Add a wrapper of wcwidth() that picks the best implementation

This adds a i_wcwidth() function that replaces mk_wcwidth(), and a
'wcwidth_implementation' setting to pick which one it wraps.

Values:

- old: uses our local mk_wcwidth() which implements unicode 5.0
- system: uses the libc-provided wcwidth(), which may be better or worse
  than ours depending on how up to date the system is.
- auto: tests the system one against two characters that became
  fullwidth in unicode 5.2 and 9.0 respectively. If either of them pass,
  pick the system implementation, otherwise pick ours.

It defaults to auto.

mk_wcwidth() is still preferable in some cases, since the way it uses
ranges for fullwidth characters means most CJK blocks are covered even
if their characters didn't exist back then.

The "system" implementation is also wrapped to never return -1, but to
assume those unknown characters use one cell. Quoting the code:

    /* Treat all unknown characters as taking one cell. This is
     * the reason mk_wcwidth and other outdated implementations
     * mostly worked with newer unicode, while glibc's wcwidth
     * needs updating to recognize new characters.
     *
     * Instead of relying on that, we keep the behavior of assuming
     * one cell even for glibc's implementation, which is still
     * highly accurate and less of a headache overall.
     */

src/core/wcwidth-wrapper.c

@@ -0,0 +1,117 @@
+/*
+ wcwidth-wrapper.c : irssi
+
+    Copyright (C) 2018 dequis
+
+    This program is free software; you can redistribute it and/or modify
+    it under the terms of the GNU General Public License as published by
+    the Free Software Foundation; either version 2 of the License, or
+    (at your option) any later version.
+
+    This program is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
+
+    You should have received a copy of the GNU General Public License along
+    with this program; if not, write to the Free Software Foundation, Inc.,
+    51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+*/
+
+#define _XOPEN_SOURCE
+#include <wchar.h>
+
+#include "module.h"
+#include "signals.h"
+#include "settings.h"
+#include "utf8.h"
+
+/* wcwidth=2 since unicode 5.2.0 */
+#define UNICODE_SQUARE_HIRAGANA_HOKA 0x1F200
+
+/* wcwidth=2 since unicode 9.0.0 */
+#define UNICODE_IRSSI_LOGO 0x1F525
+
+enum {
+	WCWIDTH_IMPL_AUTO = 0,
+	WCWIDTH_IMPL_OLD,
+	WCWIDTH_IMPL_SYSTEM,
+};
+
+WCWIDTH_FUNC wcwidth_impl_func = mk_wcwidth;
+
+int i_wcwidth(unichar ucs)
+{
+	return (*wcwidth_impl_func)(ucs);
+}
+
+static int system_wcwidth(unichar ucs)
+{
+	int retval = wcwidth((wchar_t) ucs);
+
+	if (retval < 0) {
+		/* Treat all unknown characters as taking one cell. This is
+		 * the reason mk_wcwidth and other outdated implementations
+		 * mostly worked with newer unicode, while glibc's wcwidth
+		 * needs updating to recognize new characters.
+		 *
+		 * Instead of relying on that, we keep the behavior of assuming
+		 * one cell even for glibc's implementation, which is still
+		 * highly accurate and less of a headache overall.
+		 */
+		return 1;
+	}
+
+	return retval;
+}
+
+static void read_settings(void)
+{
+	static int choice = -1;
+	int newchoice;
+
+	newchoice = settings_get_choice("wcwidth_implementation");
+
+	if (choice == newchoice) {
+		return;
+	}
+
+	choice = newchoice;
+
+	switch (choice) {
+	case WCWIDTH_IMPL_AUTO:
+		/* Test against characters that have wcwidth=2
+		 * since unicode 5.2 and 9.0 respectively */
+
+		if (system_wcwidth(UNICODE_SQUARE_HIRAGANA_HOKA) == 2 ||
+		    system_wcwidth(UNICODE_IRSSI_LOGO) == 2) {
+			wcwidth_impl_func = &system_wcwidth;
+		} else {
+			/* Fall back to our own (which implements 5.0) */
+			wcwidth_impl_func = &mk_wcwidth;
+		}
+		break;
+
+	case WCWIDTH_IMPL_OLD:
+		wcwidth_impl_func = &mk_wcwidth;
+		break;
+
+	case WCWIDTH_IMPL_SYSTEM:
+		wcwidth_impl_func = &system_wcwidth;
+		break;
+	}
+
+}
+
+void wcwidth_wrapper_init(void)
+{
+	settings_add_choice("misc", "wcwidth_implementation", WCWIDTH_IMPL_AUTO, "auto;old;system");
+
+	read_settings();
+	signal_add("setup changed", (SIGNAL_FUNC) read_settings);
+}
+
+void wcwidth_wrapper_deinit(void)
+{
+	signal_remove("setup changed", (SIGNAL_FUNC) read_settings);
+}