Change concat to cygpath throughout. Change map_filename to cygpath

Change concat to cygpath throughout.  Change map_filename to cygpath
throughout.
* concat.cc (vconcat): New function.
(concat): Use vconcat.
* concat.h: Reflect above.
* install.cc (map_filename): Eliminate.
(install_one): Free dest_file.
* mount.cc (find2): Expect input key to be pointing to mount record.
(in_table): New function.
(is_admin): Move from root.cc.
(read_mounts): New function.
(path_prefix_p): New function.
(cygpath): New function.
* mount.h: Define new functions and structure.
* root.cc (in_table): Move to mount.cc.
(do_root): Call read_mounts to initialize root stuff and mount table.
* site.cc (get_root_dir): Ditto.