Mark internal nss symbols with attribute_hidden [BZ #18822]

Mark internal nss symbols with attribute_hidden to allow direct access
within libc.so and libc.a without using GOT nor PLT.

Tested on x86-64 with and without --disable-nscd.

	[BZ #18822]
	* grp/initgroups.c (__nss_group_database): Removed.
	(__nss_initgroups_database): Likewise.
	* nscd/gai.c (__nss_hosts_database): Likewise.
	* nss/XXX-lookup.c (DATABASE_NAME_SYMBOL): Likewise.
	* posix/tst-rfc3484-2.c (__nss_hosts_database): Likewise.
	* posix/tst-rfc3484-3.c (__nss_hosts_database): Likewise.
	* posix/tst-rfc3484.c (__nss_hosts_database): Likewise.
	* sysdeps/posix/getaddrinfo.c (__nss_hosts_database): Likewise.
	* nss/getXXent.c (INTERNAL (REENTRANT_GETNAME)): Add
	attribute_hidden.
	* nss/nsswitch.c (__nss_database_custom): Define only if
	USE_NSCD is defined.
	(__nss_configure_lookup): Use __nss_database_custom only if
	USE_NSCD is defined.
	* nss/nsswitch.h (__nss_database_custom): Declare only if
	USE_NSCD is defined.  Add attribute_hidden.
	(__nss_setent): Add attribute_hidden.
	(__nss_endent): Likewise.
	(__nss_getent_r): Likewise.
	(__nss_getent): Likewise.
	(DEFINE_DATABASE): Declare __nss_##arg##_database.