(chibi filesystem): add chown

lib/chibi/filesystem.sld

@@ -33,7 +33,7 @@
           open/append      open/non-block
           file-lock        file-truncate
           file-is-readable? file-is-writable? file-is-executable?
-          chmod is-a-tty?)
+          chmod chown is-a-tty?)
   (cond-expand
    (chibi
     (export lock/shared lock/exclusive lock/non-blocking lock/unlock)

lib/chibi/filesystem.stub

@@ -267,6 +267,10 @@
 
 (define-c int chmod (string int))
 
+;;> Sets the file owner and group as in chown.
+
+(define-c int chown (string uid_t gid_t))
+
 ;;> Returns \scheme{#t} if the given port of file descriptor
 ;;> if backed by a TTY object, and \scheme{#f} otherwise.