Merge branch 'settings-references'

This adds the ability to reference existing sections to the settings parser.
Mainly for swanctl.conf, where this could simplify complex configs a lot
as redundant information has only to be specified once and may then be
included in other sections (there is an example in the man page and
there are some in the unit tests).

Also added is a new setting in filelog sections to specify the path of
the log file (in case it contains characters that are not allowed in section
names). We should encourage people to configure their log files that way
which might allow use to prohibit dots in section names in the future.