settings: Add flex/bison based parser for strongswan.conf

This parser features several improvements over the existing one.
For instance, quoted strings (with escape sequences), unlimited includes,
relaxed newline handling (e.g. at the end of files or before/after { and }),
and the difference between empty and unset values (key = vs. key = "").

It also complains a lot more about invalid syntax. The current one accepts
pretty odd stuff (like settings or sections without name) without any
errors or warnings.

src/libstrongswan/Android.mk

@@ -33,6 +33,7 @@ pen/pen.c plugins/plugin_loader.c plugins/plugin_feature.c processing/jobs/job.c
 processing/jobs/callback_job.c processing/processor.c processing/scheduler.c \
 processing/watcher.c resolver/resolver_manager.c resolver/rr_set.c \
 selectors/traffic_selector.c settings/settings.c settings/settings_types.c \
+settings/settings_parser.c settings/settings_lexer.c \
 threading/thread.c threading/thread_value.c threading/mutex.c \
 threading/semaphore.c threading/rwlock.c threading/spinlock.c \
 utils/utils.c utils/chunk.c utils/debug.c utils/enum.c utils/identification.c \