testing: Update x509-ada version to 0.1.1

Fixes #705.
testing: Update certs and keys in tkm tests

References #705.
swanctl: Document --stats command
testing: Make strongSwan build recipe more configurable
testing: Add packages to rebuild strongSwan from the repository
testing: Add a script to build the current (or an arbitrary) source tree

This allows to (relatively) quickly (re-)build and install the current
or an arbitrary strongSwan source tree within the root image.

bindfs is used to bind mount the source directory using the regular user
and group (only works if sudo is used to run the script) so that newly
created files are not owned by root.

As with building the root image in general the guests must not be
running while executing this script.  The guest images are automatically
rebuilt after the root image has been updated so configuration files and
other modifications in guests will be lost.
testing: Use multiple jobs to install strongSwan
library: Store the used root strongswan.conf configuration
settings: Use strongswan.conf used during library initialization for reload

Since 4b670a20 we require an explicit strongswan.conf to re-load configurations.
However, the define was missing in the build, breaking SIGHUP based config
reloading.

Fixes #651.
settings: Make loading a NULL or empty pattern a (nop-)success
encoding: Accept all exchange types for non IKEv1/IKEv2 major versions
vici: Add a command to reload strongswan.conf
swanctl: Add a --reload-settings command
swanctl: Add a --load-all command, performing --load-{creds,pools,conns}
swanctl: Fix description of load-pools command summary
swanctl: Complete --load-creds command summary
systemd: Provide a charon-systemd daemon targeting full systemd integration
plugin-loader: Support a reload() callback for static features
systemd: Add a native systemd journal logger
systemd: Discover and check systemd libraries with pkg-config during configure