Prevent psql from issuing BEGIN before ALTER SYSTEM when AUTOCOMMIT is off.

The autocommit-off mode works by issuing an implicit BEGIN just before
any command that is not already in a transaction block and is not itself
a BEGIN or other transaction-control command, nor a command that
cannot be executed inside a transaction block. This commit prevents psql
from issuing such an implicit BEGIN before ALTER SYSTEM because it's
not allowed inside a transaction block.

Backpatch to 9.4 where ALTER SYSTEM was added.

Report by Feike Steenbergen

src/bin/psql/common.c

@@ -1524,6 +1524,23 @@ command_no_begin(const char *query)
 		return false;
 	}
 
+	if (wordlen == 5 && pg_strncasecmp(query, "alter", 5) == 0)
+	{
+		query += wordlen;
+
+		query = skip_white_space(query);
+
+		wordlen = 0;
+		while (isalpha((unsigned char) query[wordlen]))
+			wordlen += PQmblen(&query[wordlen], pset.encoding);
+
+		/* ALTER SYSTEM isn't allowed in xacts */
+		if (wordlen == 6 && pg_strncasecmp(query, "system", 6) == 0)
+			return true;
+
+		return false;
+	}
+
 	/*
 	 * Note: these tests will match DROP SYSTEM and REINDEX TABLESPACE, which
 	 * aren't really valid commands so we don't care much. The other four