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/backend/regex/regc_lex.c

@@ -716,10 +716,10 @@ static int						/* not actually used, but convenient for RETV */
 lexescape(struct vars * v)
 {
 	chr			c;
-	static chr	alert[] = {
+	static const chr alert[] = {
 		CHR('a'), CHR('l'), CHR('e'), CHR('r'), CHR('t')
 	};
-	static chr	esc[] = {
+	static const chr esc[] = {
 		CHR('E'), CHR('S'), CHR('C')
 	};
 	const chr  *save;