cmake: fix kerberos library lookup, support Heimdal

cmake: fix kerberos library lookup, support Heimdal

A build with the gold linker broke with:

    run/libwireshark.so.0.0.0: error: undefined reference to 'krb5_c_decrypt'

Fix this my restructuring the FindKERBEROS module to use the libraries
found by pkg-config.

While at it, check for MIT and Heimdal instead of assuming MIT. Remove
HAVE_KEYTYPE_ARCFOUR_56 as this macro is not used.

Change-Id: Iab23d79bc3f25e9c0fd7203b6f050f875fb4a2b5
Reviewed-on: https://code.wireshark.org/review/10907
Petri-Dish: Peter Wu <peter@lekensteyn.nl>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Jörg Mayer <jmayer@loplof.de>
Reviewed-by: Peter Wu <peter@lekensteyn.nl>