starter: Add new bison/flex based parser for ipsec.conf

The parser simply returns key/value pairs of all sections, it already
resolves also= and allows overriding options in all included sections
(not only %default), options set in included section can also be cleared
again (key=).
It provides other improvements too, like quoted strings (with escape
sequences), unlimited includes and better whitespace/comment handling.

src/starter/.gitignore

@@ -1,4 +1,7 @@
 starter
+parser/lexer.c
+parser/parser.[ch]
+parser/parser.output
 lexer.c
 parser.h
 parser.c