Support BSD and e2fsprogs UUID libraries alongside OSSP UUID library.

Allow the contrib/uuid-ossp extension to be built atop any one of these
three popular UUID libraries.  (The extension's name is now arguably a
misnomer, but we'll keep it the same so as not to cause unnecessary
compatibility issues for users.)

We would not normally consider a change like this post-beta1, but the issue
has been forced by our upgrade to autoconf 2.69, whose more rigorous header
checks are causing OSSP's header files to be rejected on some platforms.
It's been foreseen for some time that we'd have to move away from depending
on OSSP UUID due to lack of upstream maintenance, so this is a down payment
on that problem.

While at it, add some simple regression tests, in hopes of catching any
major incompatibilities between the three implementations.

Matteo Beccati, with some further hacking by me
doc:  improve ssl_ecdh_curve descriptions

Patch by Marko Kreen
Propagate system identifier generation improvement into pg_resetxlog.

Commit 5035701e07e8bd395aa878465a102afd7b74e8c3 improved xlog.c's method
for creating a database system identifier, but I neglected to fix the
copy of that code appearing in pg_resetxlog.c.  Spotted by Andres Freund.
Fix obsolete config-module-exclusion logic in vcregress.pl.

The recent addition of regression tests to uuid-ossp exposed the fact
that the MSVC build system wasn't being consistent about whether it was
building/testing that contrib module, ie, it would try to test the module
even when it hadn't built it.  The same hazard was latent for sslinfo.

For the moment I just copied the more up-to-date logic from point A to
point B, but this is screaming for refactoring.

Per buildfarm results.
pg_lsn should not be marked typispreferred.

In general it's not a good idea for built-in types in the 'U' category
to be marked preferred; they could draw behavior away from user-defined
types with similarly-named operators.  pg_lsn is probably at low risk
of that right now given the lack of casts between it and other types,
but that doesn't make this marking OK.

Ordinarily we'd bump catversion when changing any predefined catalog
contents like this, but since we're past beta1, the costs of a forced
initdb seem to outweigh the benefits of guaranteed behavioral consistency.
There's not any known behavioral impact today anyway --- this is more
in the nature of being sure there's not problems in future.

Per an off-list complaint from Thomas Fanghaenel.
doc:  improve markup of ssl_ecdh_curve commit
Ensure cleanup in case of early errors in streaming base backups

Move the code that sends the initial status information as well as the
calculation of paths inside the ENSURE_ERROR_CLEANUP block. If this code
failed, we would "leak" a counter of number of concurrent backups, thereby
making the system always believe it was in backup mode. This could happen
if the sending failed (which it probably never did given that the small
amount of data to send would never cause a flush) or if the psprintf calls
ran out of memory. Both are very low risk, but all operations after
do_pg_start_backup should be protected.