Declare tcgetsid for XPG4 (bug 20055).

termios.h should declare tcgetsid for XPG4, but only does so for
__USE_UNIX98 || __USE_XOPEN2K8 at present.  This patch fixes the
declaration conditions.  A spurious conformtest expectation of this
declaration for XPG3 is removed, and two XFAILs that are fixed by
these changes are also removed.

Tested for x86_64 and x86 (testsuite, and that installed stripped
shared libraries are unchanged by the patch).

	[BZ #20055]
	* termios/termios.h (pid_t): Define for [__USE_XOPEN_EXTENDED]
	instead of [__USE_UNIX98].
	(tcgetsid): Declare for [__USE_XOPEN_EXTENDED] instead of
	[__USE_UNIX98].
	* conform/data/termios.h-data (tcgetsid): Do not expect for
	[XPG3].
	* conform/Makefile (test-xfail-XPG3/termios.h/conform): Remove
	variable.
	(test-xfail-XPG4/termios.h/conform): Likewise.