[BUGFIX] Reports module: Available services broken

Displaying the available services in the reports module is broken.
This is a regression due to cleaning up the global declarations for
services ($T3_SERVICES) in TYPO4 4.6.

Because of this method t3lib_extMgm::findService cannot be abused
into getting a service by its key explicitly. Hence a new method must
be introduced and used in the available services report.

This patch also introduces method t3lib_extMgm::isServiceAvailable
which refactors part of the services-related code to avoid
duplication.

Change-Id: I15fd7eecaafb52df0599598c5b700e458e63f0d9
Resolves: #30554
Releases: 4.6
Reviewed-on: http://review.typo3.org/5538
Reviewed-by: Lorenz Ulrich
Tested-by: Lorenz Ulrich
Reviewed-by: Steffen Gebert
Tested-by: Steffen Gebert