spinlock.h: Fix pthread_sigmask() pointer aliasing issue.

POSIX specifies that the two sigset_t * arguments to pthread_sigmask()
are not allowed to alias.  Such a restriction would permit implementing
pthread_sigmask() by first copying the current signal mask into 'oldset'
and only then reading 'set', and AIX 5.2 and AIX 7 indeed seem to be
doing something like that.

However, spinlock.h's spin_lock_sigmask() would call pthread_sigmask()
with the same sigset_t pointer for both arguments, causing this call
to fail to mask signals entirely, which would lead to a deadlock seen
in production use due to a recursive lock on the sig_interests_lock
from a signal handler for a signal received while preparing to fork.

Fix this by using a separate sigset_t for the 'set' argument.

Signed-off-by: Lennert Buytenhek <buytenh@wantstofly.org>
Bump version number to 0.30.6.

Signed-off-by: Lennert Buytenhek <buytenh@wantstofly.org>