wiretap: add a parameter to wtap_init() indicating whether plugins must be loaded

wiretap: add a parameter to wtap_init() indicating whether plugins must be loaded

g995812c5f1 moved wiretap plugins registration from applications to
wiretap library init function.
As we do not want to load plugins for all users of libwiretap, let's
make it configurable.

Bug: 14314
Change-Id: Id8fdcc484e2d0d31d3ab0bd357d3a6678570f700
Reviewed-on: https://code.wireshark.org/review/25194
Reviewed-by: Dario Lombardo <lomato@gmail.com>
Petri-Dish: Dario Lombardo <lomato@gmail.com>
Tested-by: Petri Dish Buildbot
Reviewed-by: Anders Broman <a.broman58@gmail.com>