Use appendStringInfoString/Char et al where appropriate.

Patch by David Rowley. Backpatch to 9.5, as some of the calls were new in
9.5, and keeping the code in sync with master makes future backpatching
easier.

src/bin/pg_basebackup/pg_basebackup.c

@@ -1516,7 +1516,7 @@ GenerateRecoveryConf(PGconn *conn)
 
 		/* Separate key-value pairs with spaces */
 		if (conninfo_buf.len != 0)
-			appendPQExpBufferStr(&conninfo_buf, " ");
+			appendPQExpBufferChar(&conninfo_buf, ' ');
 
 		/*
 		 * Write "keyword=value" pieces, the value string is escaped and/or