Use pthread_sigmask() in place of sigprocmask(), for thread safety.

Use pthread_sigmask() in place of sigprocmask(), for thread safety.

POSIX says that multithreaded programs must not use sigprocmask() but must
use pthread_sigmask() instead.  This commit makes that replacement.

The actual use of signals in Open vSwitch is still not thread safe
following this commit, but this change is a necessary prerequisite for
fixing the other problems.

Signed-off-by: Ben Pfaff <blp@nicira.com>