sort: use pthread_sigmask, not sigprocmask

sort: use pthread_sigmask, not sigprocmask

POSIX says sigprocmask has unspecified behavior in a multithreaded
program like ‘sort’.
* src/sort.c (pthread_sigmask) [GNULIB_defined_pthread_functions]:
New macro, for use when ‘sort’ is not multithreaded.
(cs_enter, cs_leave): Use it.  Pass address, not value, as
this is typically a tad faster.  All callers changed.