Support styling on native MS-Windows console

Support styling on native MS-Windows console

gdb/ChangeLog:
2019-03-08  Eli Zaretskii  <eliz@gnu.org>

	PR/24315
	* utils.c (can_emit_style_escape) [_WIN32]: Don't disable styling
	on MS-Windows if $TERM is not defined.

	* cli/cli-style.c: Set cli_styling to 1 in the MinGW build.

	* posix-hdep.c (gdb_console_fputs):
	* mingw-hdep.c (rgb_to_16colors, gdb_console_fputs): New
	functions.
	* ui-file.h (gdb_console_fputs): Add prototype.

	* ui-file.c (stdio_file::puts): Call gdb_console_fputs, and fall
	back to fputs only if the former returns zero.