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/tsearch/wparser_def.c

@@ -789,7 +789,7 @@ p_isspecial(TParser *prs)
 	 */
 	if (GetDatabaseEncoding() == PG_UTF8 && prs->usewide)
 	{
-		static pg_wchar strange_letter[] = {
+		static const pg_wchar strange_letter[] = {
 			/*
 			 * use binary search, so elements should be ordered
 			 */
@@ -1023,7 +1023,7 @@ p_isspecial(TParser *prs)
 			0xAA34,				/* CHAM CONSONANT SIGN RA */
 			0xAA4D				/* CHAM CONSONANT SIGN FINAL H */
 		};
-		pg_wchar   *StopLow = strange_letter,
+		const pg_wchar *StopLow = strange_letter,
 				   *StopHigh = strange_letter + lengthof(strange_letter),
 				   *StopMiddle;
 		pg_wchar	c;