CacheManager: require /squid-internal-mgr/ URL path prefix (#1426)

CacheManager: require /squid-internal-mgr/ URL path prefix (#1426)

    ERROR: Squid BUG: assurance failed: tok.skip(internalMagicPrefix)
    exception location: cache_manager.cc(173) ParseUrl

Due to poor code duplication, commit 92a5adb accidentally classified
URLs without a trailing slash in the magical prefix as valid cache
manager URLs, triggering the above ERRORs. We were denying such
"slashless" cache manager URLs (as invalid internal URLs) prior to that
commit. Since that commit, the ERRORs triggered by that commit
effectively denied them as well. Denying them properly results in
simpler/smaller code (than allowing them would), so we should avoid a UI
change and continue to deny them, at least for now.

This change also reduces duplication of magic prefix definitions. Other
pending work will completely eliminate that duplication in src/ code.