utils: document p{open,close}_re_clearsigmask() requires locking.

Signed-off-by: Andrey Mazo <mazo@telum.ru>
utils: nicer names for popen_re_clearsigmask()/pclose_re_clearsigmask().

Also convert all existing users to the new names.

Signed-off-by: Andrey Mazo <mazo@telum.ru>