Few cleanups and couple of new things:

 - add SHA2 algorithm to older OpenSSL
 - add BIGNUM math to have public-key cryptography work on non-OpenSSL
   build.
 - gen_random_bytes() function

The status of SHA2 algoritms and public-key encryption can now be
changed to 'always available.'

That makes pgcrypto functionally complete and unless there will be new
editions of AES, SHA2 or OpenPGP standards, there is no major changes
planned.

  fe-auth.c:573: warning: passing argument 1 of 'free' discards qualifiers
  from pointer target type
pg_krb5_authname used to return a (const char *) to memory allocated by
krb. Somewhere along the lines this was changed so that a copy was
made, returned, and freed instead. However the const modifier was never
removed.

- halt.c did not include stdlib.h, thus missed exit() prototype
- Makefile ignores BINDIR for install.
- Makefile calls install with user/group args, thus failing for regular user.

While trying it I noticed that the Makefile does not support VPATH builds ...

it can handle small fillfactors for ordinary-sized index entries without
failing on large ones; fix nbtinsert.c to distinguish leaf and nonleaf
pages; change the minimum fillfactor to 10% for all index types.

SGML cleanup: sort table entries alphabetically.

- Replace sorted array of entries in maintenance_work_mem to binary tree,
  this should improve create performance.
- More precisely calculate allocated memory, eliminate leaks
  with user-defined extractValue()
- Improve wordings in tsearch2

Thank to Mario Weilguni <mweilguni@sime.com> to discover a bug.

header files.

a table.  Otherwise a USING clause that yields NULL can leave the table
violating its constraint (possibly there are other cases too).  Per report
from Alexander Pravking.

To this end, add a couple of columns to pg_class, relminxid and relvacuumxid,
based on which we calculate the pg_database columns after each vacuum.

We now force all databases to be vacuumed, even template ones.  A backend
noticing too old a database (meaning pg_database.datminxid is in danger of
falling behind Xid wraparound) will signal the postmaster, which in turn will
start an autovacuum iteration to process the offending database.  In principle
this is only there to cope with frozen (non-connectable) databases without
forcing users to set them to connectable, but it could force regular user
database to go through a database-wide vacuum at any time.  Maybe we should
warn users about this somehow.  Of course the real solution will be to use
autovacuum all the time ;-)

There are some additional improvements we could have in this area: for example
the vacuum code could be smarter about not updating pg_database for each table
when called by autovacuum, and do it only once the whole autovacuum iteration
is done.

I updated the system catalogs documentation, but I didn't modify the
maintenance section.  Also having some regression tests for this would be nice
but it's not really a very straightforward thing to do.

Catalog version bumped due to system catalog changes.