Further atomic ops portability improvements and bug fixes.

* Don't play tricks for a more efficient pg_atomic_clear_flag() in the
  generic gcc implementation. The old version was broken on gcc < 4.7
  on !x86 platforms. Per buildfarm member chipmunk.
* Make usage of __atomic() fences depend on HAVE_GCC__ATOMIC_INT32_CAS
  instead of HAVE_GCC__ATOMIC_INT64_CAS - there's platforms with 32bit
  support that don't support 64bit atomics.
* Blindly fix two superflous #endif in generic-xlc.h
* Check for --disable-atomics in platforms but x86.
Fix identify_locking_dependencies for schema-only dumps.

Without this fix, parallel restore of a schema-only dump can deadlock,
because when the dump is schema-only, the dependency will still be
pointing at the TABLE item rather than the TABLE DATA item.

Robert Haas and Tom Lane
Fix relcache for policies, and doc updates

Andres pointed out that there was an extra ';' in equalPolicies, which
made me realize that my prior testing with CLOBBER_CACHE_ALWAYS was
insufficient (it didn't always catch the issue, just most of the time).
Thanks to that, a different issue was discovered, specifically in
equalRSDescs.  This change corrects eqaulRSDescs to return 'true' once
all policies have been confirmed logically identical.  After stepping
through both functions to ensure correct behavior, I ran this for
about 12 hours of CLOBBER_CACHE_ALWAYS runs of the regression tests
with no failures.

In addition, correct a few typos in the documentation which were pointed
out by Thom Brown (thanks!) and improve the policy documentation further
by adding a flushed out usage example based on a unix passwd file.

Lastly, clean up a few comments in the regression tests and pg_dump.h.
Implement pgp_armor_header()
Fix variable declarations
Add test case for extracting a comment
Add support for generating headers in armor()
Add tests for non-1-indexed arrays (yuck)
Add docs
Add pgcrypto--1.2.sql
Add pgcrypto--1.1--1.2.sql
Fix comment in test case
Fix comment
Fix compiler warning
Make UTF-8 handling more consistent
Replace some spaces with tabs
Fix mistakes in merge conflict resolution
Use appendStringInfoChar instead of the macro version
Small fixes
Use StringInfo instead of mbuf