rename __sa_restorer to sa_restorer in struct sigaction

this is legal since sa_* is in the reserved namespace for signal.h,
per posix. note that the sa_restorer field is not used anywhere, so
programs that are trying to use it may still break, but at least
they'll compile. if it turns out such programs actually need to be
able to set their own sa_restorer to function properly, i'll add the
necessary code to sigaction.c later.