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

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.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/4039c736eb0955cb1daf88e211f105dbbb78f7ea

Modified Files
--------------
src/backend/storage/lmgr/spin.c | 30 ++++++++++++++++++++++--------
1 file changed, 22 insertions(+), 8 deletions(-)

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