Increase robustness of internal dlopen() by using RTLD_NOW [BZ #22766]

Prevent random runtime crashes due to missing symbols caused by mixed
libnss_* versions.

	[BZ #22766]
	* include/dlfcn.h [__libc_dl_open]: Replace RTLD_LAZY with RTLD_NOW.
	* sysdeps/gnu/unwind-resume.c (__lib_gcc_s_init): Replace
	__libc_dlopen_mode() using RTLD_NOW with __libc_dlopen.
	* sysdeps/nptl/unwind-forcedunwind.c: Likewise.

Signed-off-by: Tulio Magno Quites Machado Filho <tuliom@linux.ibm.com>
Reviewed-by: Carlos O'Donell <carlos@redhat.com>