sigprocmask should ignore HOW argument when SET is NULL.

sigprocmask should ignore HOW argument when SET is NULL.

Specific use case bug found in SysRes VG_(do_sys_sigprocmask).

Fix for case when ,,set,, parameter is NULL.
In this case ,,how,, parameter should be ignored because we are
only requesting from kernel to put current signal mask into ,,oldset,,.
But instead we determine the action based on ,,how,, parameter and
therefore make the system call fail when it should pass.
Taken from linux man pages (sigprocmask).

The same is specified for POSIX.

https://bugs.kde.org/show_bug.cgi?id=414565