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.