pthread_sigmask: fix bug on FreeBSD 9

pthread_sigmask: fix bug on FreeBSD 9

* lib/pthread_sigmask.c [PTHREAD_SIGMASK_INEFFECTIVE]:
Include string.h.
(pthread_sigmask) [PTHREAD_SIGMASK_INEFFECTIVE]:
When calling pthread_sigmask (1729, NEW, OLD), specify non-null NEW;
this avoids a bug on FreeBSD 9, where pthread_sigmask is effective
but pthread_sigmask (1729, NULL, NULL) returns zero.
See <http://bugs.gnu.org/11884>.
Avoid the need to call pthread_sigmask (1729, ...) in most cases,
by inspecting whether the main call changed the old mask.