maint: revert “avoid pthread_sigmask lock”

maint: revert “avoid pthread_sigmask lock”

* configure.ac (GNULIB_SIGACTION_SINGLE_THREAD): Remove.
This never worked (it was a misspelling) and the properly-spelled
identifier (whose spelling has since been renamed) is useful
mostly for programs like gzip that do not need Gnulib’s ‘lock’ module.
For coreutils, which needs ‘lock’ for other reasons, it’s overkill.

maint: avoid pthread_sigmask lock overhead
This matters only for MS-Windows.
* configure.ac (GNULIB_PTHREAD_SIGMASK_SINGLE_THREAD):
Define this instead of defining GNULIB_SIGACTION_SINGLE_THREAD.
The latter was a typo, and Gnulib has evolved anyway.