Make various variables const (read-only).

These changes should generally improve correctness/maintainability.
A nice side benefit is that several kilobytes move from initialized
data to text segment, allowing them to be shared across processes and
probably reducing copy-on-write overhead while forking a new backend.
Unfortunately this doesn't seem to help libpq in the same way (at least
not when it's compiled with -fpic on x86_64), but we can hope the linker
at least collects all nominally-const data together even if it's not
actually part of the text segment.

Also, make pg_encname_tbl[] static in encnames.c, since there seems
no very good reason for any other code to use it; per a suggestion
from Wim Lewis, who independently submitted a patch that was mostly
a subset of this one.

Oskari Saarenmaa, with some editorialization by me

src/common/relpath.c

@@ -29,7 +29,7 @@
  * documentation for pg_relation_size(). Also keep FORKNAMECHARS above
  * up-to-date.
  */
-const char *forkNames[] = {
+const char *const forkNames[] = {
 	"main",						/* MAIN_FORKNUM */
 	"fsm",						/* FSM_FORKNUM */
 	"vm",						/* VISIBILITYMAP_FORKNUM */