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
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 <firstname.lastname@example.org>