Rename windows-termcap.c -> stub-termcap.c

Rename windows-termcap.c -> stub-termcap.c

Preparation for using this on all hosts.

Confirmed that --host=x86_64-w64-mingw32 still builds the stub
termcap.

gdb/ChangeLog:
2015-02-26  Pedro Alves  <palves@redhat.com>

	* windows-termcap.c: Rename to ...
	* stub-termcap.c: ... this.  Adjust header line.
	* Makefile.in (SFILES): Refer to stub-termcap.c instead of
	windows-termcap.c.
	* configure: Regenerate.
	* configure.ac: Refer to stub-termcap.o instead of
	windows-termcap.o.
	* gdb_curses.h: Mention stub-termcap.c instead of
	windows-termcap.c.