change sigset_t functions to restrict to _NSIG

the idea here is to avoid advertising signals that don't exist and to
make these functions safe to call (e.g. from within other parts of the
implementation) on fake sigset_t objects which do not have the HURD
padding.

fix off-by-one error in checks for implementation-internal signal numbers

overhaul implementation-internal signal protections

the new approach relies on the fact that the only ways to create
sigset_t objects without invoking UB are to use the sig*set()
functions, or from the masks returned by sigprocmask, sigaction, etc.
or in the ucontext_t argument to a signal handler. thus, as long as
sigfillset and sigaddset avoid adding the "protected" signals, there
is no way the application will ever obtain a sigset_t including these
bits, and thus no need to add the overhead of checking/clearing them
when sigprocmask or sigaction is called.

note that the old code actually *failed* to remove the bits from
sa_mask when sigaction was called.

the new implementations are also significantly smaller, simpler, and
faster due to ignoring the useless "GNU HURD signals" 65-1024, which
are not used and, if there's any sanity in the world, never will be
used.

initial check-in, version 0.5.0