increase limit on locale name length from 15 to 23 bytes

the old limit was one byte too short to support locale names of the
form xx_XX.UTF-8@modifier where modifier is more than 3 bytes, a form
which various real-world locale names take. the problem could be
avoided by omitting the useless ".UTF-8" part, but users may need to
have it present when operating on mixed-libc systems or when it will
be carried over (e.g. across ssh) to other systems.

the new limit is chosen sufficient for existing/reasonable locale
names while still keeping the size of setlocale's static buffer small.

also add locale_impl.h to the Makefile's list of headers which force
rebuild of source files, to prevent dangerously inconsistent object
files from getting used after this change.

Makefile

@@ -35,7 +35,7 @@ AOBJS = $(LIBC_OBJS)
 LOBJS = $(LIBC_OBJS:.o=.lo)
 GENH = obj/include/bits/alltypes.h obj/include/bits/syscall.h
 GENH_INT = obj/src/internal/version.h
-IMPH = $(addprefix $(srcdir)/, src/internal/stdio_impl.h src/internal/pthread_impl.h src/internal/libc.h)
+IMPH = $(addprefix $(srcdir)/, src/internal/stdio_impl.h src/internal/pthread_impl.h src/internal/locale_impl.h src/internal/libc.h)
 
 LDFLAGS =
 LDFLAGS_AUTO =

src/internal/locale_impl.h

@@ -6,7 +6,7 @@
 #include "libc.h"
 #include "pthread_impl.h"
 
-#define LOCALE_NAME_MAX 15
+#define LOCALE_NAME_MAX 23
 
 struct __locale_map {
 	const void *map;