Mark internal grp/pwd/shadow functions with attribute_hidden [BZ #18822]

Mark internal grp/pwd/shadow functions with attribute_hidden to allow
direct access within libc.so and libc.a without using GOT nor PLT.

	[BZ #18822]
	* include/grp.h (__fgetgrent_r): Add attribute_hidden.
	(__getgrgid_r): Likewise.
	(__getgrnam_r): Likewise.
	* include/pwd.h (__getpwuid_r): Likewise.
	(__getpwnam_r): Likewise.
	(__fgetpwent_r): Likewise.
	* include/shadow.h (__getspnam_r): Likewise.
	(__sgetspent_r): Likewise.
	(__fgetspent_r): Likewise.