Merge branch 'swanctl-plugins'

Uses a separate default plugin list for swanctl (instead of just loading
all of libstrongswan's plugins) to avoid some side-effects of loaded but
unused plugins.  The load statements in the regression tests have been
updated accordingly.

A new configure option for maintainers of distributions that ship
plugins in separate packages changes the message if a plugin is not
found and reduces its log level.  This confused users of tools that don't
use modular plugin loading (e.g. pki or swanctl).

Also changes command line handling in pki and swanctl so that the shared
options can be passed before the actual command and that the debug level
also affects library/plugin initialization.

References strongswan/strongswan#2971