proposal: Use flags to select/match proposals

proposal: Use flags to select/match proposals

During proposal selection with ike/child_cfgs a couple of boolean
variables can be set (e.g. private, prefer_self, strip_dh). To simplify
the addition of new parameters, these functions now use a set of flags
instead of indiviual boolean values.

Signed-off-by: Thomas Egerer <thomas.egerer@secunet.com>