Adjust spin.c's spinlock emulation so that 0 is not a valid spinlock value.

Adjust spin.c's spinlock emulation so that 0 is not a valid spinlock value.

We've had repeated troubles over the years with failures to initialize
spinlocks correctly; see 6b93fcd14 for a recent example.  Most of the time,
on most platforms, such oversights can escape notice because all-zeroes is
the expected initial content of an slock_t variable.  The only platform
we have where the initialized state of an slock_t isn't zeroes is HPPA,
and that's practically gone in the wild.  To make it easier to catch such
errors without needing one of those, adjust the --disable-spinlocks code
so that zero is not a valid value for an slock_t for it.

In passing, remove a bunch of unnecessary #include's from spin.c;
commit daa7527afc227443 removed all the intermodule coupling that
made them necessary.