Improve behavior of tsearch_readline(), and remove t_readline().

Commit fbeb9da22, which added the tsearch_readline APIs, left
t_readline() in place as a compatibility measure.  But that function
has been unused and deprecated for twelve years now, so that seems
like enough time to remove it.  Doing so, and merging t_readline's
code into tsearch_readline, aids in making several useful
improvements:

* The hard-wired 4K limit on line length in tsearch data files is
removed, by using a StringInfo buffer instead of a fixed-size buffer.

* We can buy back the per-line palloc/pfree added by 3ea7e9550
in the common case where encoding conversion is not required.

* We no longer need a separate pg_verify_mbstr call, as that
functionality was folded into encoding conversion some time ago.

(We could have done some of this stuff while keeping t_readline as a
separate API, but there seems little point, since there's no reason
for anyone to still be using t_readline directly.)

Discussion: https://postgr.es/m/48A4FA71-524E-41B9-953A-FD04EF36E2E7@yesql.se

src/include/tsearch/ts_locale.h

@@ -15,6 +15,7 @@
 #include <ctype.h>
 #include <limits.h>
 
+#include "lib/stringinfo.h"
 #include "mb/pg_wchar.h"
 #include "utils/pg_locale.h"
 
@@ -33,7 +34,9 @@ typedef struct
 	FILE	   *fp;
 	const char *filename;
 	int			lineno;
-	char	   *curline;
+	StringInfoData buf;			/* current input line, in UTF-8 */
+	char	   *curline;		/* current input line, in DB's encoding */
+	/* curline may be NULL, or equal to buf.data, or a palloc'd string */
 	ErrorContextCallback cb;
 } tsearch_readline_state;
 
@@ -57,6 +60,4 @@ extern bool tsearch_readline_begin(tsearch_readline_state *stp,
 extern char *tsearch_readline(tsearch_readline_state *stp);
 extern void tsearch_readline_end(tsearch_readline_state *stp);
 
-extern char *t_readline(FILE *fp);
-
 #endif							/* __TSLOCALE_H__ */