httpd: Refactor handler instantiation

AliasEntry was tightly coupled to HttpServiceCell (apparent in that a admin
shell Args instance was passed to the AliasEntry factory method, and in that
several getters of HttpServiceCell were only used by the AliasEntry factory).

This patch moves the alias and handler creation into HttpServiceCell, thus
reducing coupling of AliasEntry.

The patch also simplifies the AliasType enum.

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Albert Rossi <arossi@fnal.gov>
Patch: https://rb.dcache.org/r/7423/