Instead of returning -1, return errno on error.
Catch up after the fix in libpthread by Andrew Doran in 2008
in lib/libpthread/pthread_misc.c r.1.9.
It's an open question whether this function shall be used without linked
in the POSIX thread library.
Detected by Bruno Haible (GNU) and documented in gnulib in commit
"pthread_sigmask: Avoid test failure on NetBSD 8.0. " r. 4d16a83b0c1fcb6c.
e4e4859de1