Add psql PROMPT variable for search_path.

The new %S substitution shows the current value of search_path.
Note that this only works when connected to Postgres v18 or newer,
since search_path was first marked as GUC_REPORT in commit
28a1121fd9.  On older versions that don't report search_path, %S is
replaced with a question mark.

Suggested-by: Lauri Siltanen <lauri.siltanen@gmail.com>
Author: Florents Tselai <florents.tselai@gmail.com>
Reviewed-by: Jelte Fennema-Nio <postgres@jeltef.nl>
Reviewed-by: Jim Jones <jim.jones@uni-muenster.de>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://postgr.es/m/CANsM767JhTKCRagTaq5Lz52fVwLPVkhSpyD1C%2BOrridGv0SO0A%40mail.gmail.com

src/bin/psql/prompt.c

@@ -34,6 +34,7 @@
  * %P - pipeline status: on, off or abort
  * %> - database server port number
  * %n - database user name
+ * %S - search_path
  * %s - service
  * %/ - current database
  * %~ - like %/ but "~" when database name equals user name
@@ -167,6 +168,16 @@ get_prompt(promptStatus_t status, ConditionalStack cstack)
 					if (pset.db)
 						strlcpy(buf, session_username(), sizeof(buf));
 					break;
+					/* search_path */
+				case 'S':
+					if (pset.db)
+					{
+						const char *sp = PQparameterStatus(pset.db, "search_path");
+
+						/* Use ? for versions that don't report search_path. */
+						strlcpy(buf, sp ? sp : "?", sizeof(buf));
+					}
+					break;
 					/* service name */
 				case 's':
 					{