Add frontend for fuzzing

Use the following configure command:

$ ./configure --with-fuzzer --with-fuzzer-lib=/path/to/libFuzzer.a \
      CC=clang CXX=clang++

Places an irssi-fuzz in src/fe-fuzz/ after build.

Also can specify SANFLAGS to override the chosen sanitizer flags
(defaults to "-g -fsanitize=address -fsanitize-coverage=trace-pc-guard").

src/fe-fuzz/Makefile.am

@@ -0,0 +1,25 @@
+bin_PROGRAMS = irssi-fuzz
+
+# Force link with clang++ for libfuzzer support
+CCLD=clang++ $(CXXFLAGS)
+
+AM_CPPFLAGS = \
+	-I$(top_srcdir)/src \
+	-I$(top_srcdir)/src/core/ \
+	-I$(top_srcdir)/src/irc/core/ \
+	-I$(top_srcdir)/src/fe-common/core/ \
+	$(GLIB_CFLAGS)
+
+irssi_fuzz_DEPENDENCIES = @COMMON_LIBS@
+
+irssi_fuzz_LDADD = \
+	@COMMON_LIBS@ \
+	@PROG_LIBS@ \
+	$(FUZZER_LIBS)
+
+irssi_fuzz_SOURCES = \
+        irssi.c \
+	module-formats.c
+
+noinst_HEADERS = \
+	module-formats.h