cvsserver: nested directory creation fixups for Eclipse clients

To create nested directories without (or before) sending file entries
is rather tricky. Most clients just work. Eclipse, however, expects
a very specific sequence of events. With this patch, cvsserver meets
those expectations.

Note: we may want to reuse prepdir() in req_update -- should move it
outside of req_co. Right now prepdir() is tied to how req_co() works.

Signed-off-by: Junio C Hamano <junkio@cox.net>