Fix relocation of directories in the MinGW build.

Fix relocation of directories in the MinGW build.

 windows-nat.c (windows_get_absolute_argv0): New function.
 windows-nat.h: Add its 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.
 Include windows-nat.h.