Add \ir command to psql.

\ir is short for "include relative"; when used from a script, the
supplied pathname will be interpreted relative to the input file,
rather than to the current working directory.

Gurjeet Singh, reviewed by Josh Kupershmidt, with substantial further
cleanup by me.

src/port/path.c

@@ -74,6 +74,17 @@ skip_drive(const char *path)
 #define skip_drive(path)	(path)
 #endif
 
+/*
+ *	has_drive_prefix
+ *
+ * Return true if the given pathname has a drive prefix.
+ */
+bool
+has_drive_prefix(const char *path)
+{
+	return skip_drive(path) != path;
+}
+
 /*
  *	first_dir_separator
  *