dcache-srm: Move SRM specific dCache code to new module

Makes the depedency between WebDAV and SRM explicit.

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: https://rb.dcache.org/r/7214/