Fix ttyslot namespace (bug 18547).

Fix ttyslot namespace (bug 18547).

ttyslot (XPG4) calls the non-XPG4 functions endttyent, getttyent and
setttyent, which in turn bring in references to fgets_unlocked and
getttynam.  This patch fixes this by making these functions into weak
aliases and calling the __* names as needed.

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

	[BZ #18547]
	* misc/getttyent.c (getttynam): Rename to __getttynam and define
	as weak alias of __getttynam.  Use prototype function definition.
	Call __setttyent, __getttyent and __endttyent instead of
	setttyent, getttyent and endttyent.
	(getttyent): Rename to __getttyent and define as weak alias of
	__getttyent.  Call __setttyent instead of setttyent.  Call
	__fgets_unlocked instead of fgets_unlocked.
	(setttyent): Rename to __setttyent and define as weak alias of
	__setttyent.
	(endttyent): Rename to __endttyent and define as weak alias of
	__endttyent.
	* include/ttyent.h (__getttyent): Declare.  Use libc_hidden_proto.
	(__setttyent): Likewise.
	(__endttyent): Likewise.
	(getttyent): Don't use libc_hidden_proto.
	(setttyent): Likewise.
	(endttyent): Likewise.
	* misc/ttyslot.c (ttyslot): Call __setttyent, __getttyent and
	__endttyent instead of setttyent, getttyent and endttyent.
	* conform/Makefile (test-xfail-XPG4/unistd.h/linknamespace):
	Remove variable.