daemon: extend user-relative path notation.

daemon: extend user-relative path notation.

Earlier, we made --base-path to automatically forbid
user-relative paths, which was probably a mistake.  This
introduces --user-path (or --user-path=path) option to control
the use of user-relative paths independently.  The latter form
of the option can be used to restrict accesses to a part of each
user's home directory, similar to "public_html" some webservers
supports.

If we're invoked with --user-path=FOO option, then a URL of the
form git://~USER/PATH/... resolves to the path HOME/FOO/PATH/...,
where HOME is USER's home directory.

[jc: This is much reworked by me so bugs are mine, but the
 original patch was done by Mark Wooding.]

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