This makes merging and diffing with upstream easier. There were so many
GPDB-added GUCs in guc.c that "git merge" didn't handle it very well, and
you easily got merge conflicts that were error-prone to fix manually.

I kept a few GUCs in guc.c that are not present in REL8_2_0, because they
were backported from later PostgreSQL versions, rather than being
completely GPDB-specific.

Clean up the diff between guc.c and upstream, by removing unnecessary
includes, reordering a few GUCs to match upstream better, and similar
cosmetic changes.

I had to move MAX_KILOBYTES from guc.c to guc_tables.h, because guc_gp.c
needs it too. I also had to export assign_msglvl() function from guc.c, so
that it can be called from other assign_* functions from guc_gp.c. And I
had to add the code to process the ConfigureNames*_gp arrays in addition
to the upstream ones. These things increase the diff against upstream, but
not by too much, IMHO. I also copy-pasted constants like MS_PER_S and
KB_PER_MB, which seems safe as those are natural constants, and should
never change.

Now that we no longer forcibly compile strlcpy.c, and only compile and link
it into strlcpy.o into libpgport when needed, this is no longer needed.
Furthermore, it caused the linking to fail on platforms that don't need
strlcpy.o, like OS X.