psql: Remove inappropriate const qualifiers

Since mbvalidate() can alter the string it validates, having the
callers claim that the strings they accept are const is inappropriate.

src/bin/psql/mbprint.c

@@ -385,7 +385,7 @@ unsigned char *
 mbvalidate(unsigned char *pwcs, int encoding)
 {
 	if (encoding == PG_UTF8)
-		mb_utf_validate((unsigned char *) pwcs);
+		mb_utf_validate(pwcs);
 	else
 	{
 		/*

src/bin/psql/print.c

@@ -2122,7 +2122,7 @@ printTableInit(printTableContent *const content, const printTableOpt *opt,
  * column.
  */
 void
-printTableAddHeader(printTableContent *const content, const char *header,
+printTableAddHeader(printTableContent *const content, char *header,
 					const bool translate, const char align)
 {
 #ifndef ENABLE_NLS
@@ -2162,7 +2162,7 @@ printTableAddHeader(printTableContent *const content, const char *header,
  * Note: Automatic freeing of translatable strings is not supported.
  */
 void
-printTableAddCell(printTableContent *const content, const char *cell,
+printTableAddCell(printTableContent *const content, char *cell,
 				  const bool translate, const bool mustfree)
 {
 #ifndef ENABLE_NLS

src/bin/psql/print.h

@@ -162,9 +162,9 @@ extern void printTableInit(printTableContent *const content,
 			   const printTableOpt *opt, const char *title,
 			   const int ncolumns, const int nrows);
 extern void printTableAddHeader(printTableContent *const content,
-				 const char *header, const bool translate, const char align);
+				 char *header, const bool translate, const char align);
 extern void printTableAddCell(printTableContent *const content,
-				const char *cell, const bool translate, const bool mustfree);
+				char *cell, const bool translate, const bool mustfree);
 extern void printTableAddFooter(printTableContent *const content,
 					const char *footer);
 extern void printTableSetFooter(printTableContent *const content,