magit-emacsclient-executable: discover emacsclient name via pattern match

It's possible to configure Emacs with --program-suffix=-suffix which
results in the emacsclient executable being named
emacsclient-suffix. For example, it's not uncommon to track the Emacs
development repository and configure Emacs with --program-suffix=-git or
--program-suffix=-bzr.

The current search patterns all assume Emacs wasn't configured with a
program suffix. Fix this by searching the `invocation-directory' for any
program that matches the regexp: `emacsclient.*'.