Improve MinGW support in Readline

Improve MinGW support in Readline

These changes were already accepted upstream in Readline,
but GDB did not yet import a newer Readline version.

readline/Changelog.gdb:

	* util.c: Include rlshell.h.
	(_rl_tropen) [_WIN32 && !__CYGWIN__]: Open the trace file in the
	user's temporary directory.
	* tcap.h [HAVE_NCURSES_TERMCAP_H]: Include ncurses/termcap.h.
	* input.c (w32_isatty) [_WIN32 && !__CYGWIN__]: New function, to
	replace isatty that is not reliable enough on MS-Windows.
	(isatty) [_WIN32 && !__CYGWIN__]: Redirect to w32_isatty.
	(rl_getc): Call _getch, not getch, which could be an ncurses
	function when linked with ncurses, in which case getch will return
	EOF for any keystroke, because there's no curses window.
	* tilde.c (tilde_expand_word) [_WIN32]:
	* histfile.c (history_filename) [_WIN32]: Windows-specific
	environment variable to replace HOME if that is undefined.
	* funmap.c (default_funmap): Compile rl_paste_from_clipboard on
	all Windows platforms, not just Cygwin.
	* readline.h (rl_paste_from_clipboard): Include declaration for
	all Windows platforms.
	* display.c (insert_some_chars, delete_chars): Don't use the
	MinGW-specific code if linked with ncurses.
	* configure.in:
	* config.h.in: Support ncurses/termcap.h.  The configure script
	was updated accordingly.
	* complete.c [_WIN32 && !__CYGWIN__]: Initialize
	_rl_completion_case_fold to 1.
	(printable_part, rl_filename_completion_function)
	[_WIN32 && !__CYGWIN__]: Handle the drive letter.