Fix GDB relocation on MinGW.

	* mingw-hdep.c (windows_get_absolute_argv0): New function.
	Include main.h.

	* main.h (windows_get_absolute_argv0): Add prototype.

	* main.c (get_init_files): Use filename_ncmp instead of strncmp.
	Use IS_DIR_SEPARATOR instead of looking for a character inside
	SLASH_STRING.  Include filenames.h.
	(captured_main) [__MINGW32__]: Make argv[0] absolute, so that
	relocate_gdb_directory works when passed gdb_program_name.
Fix date of last entry.
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