Add support for SCTP packet parsing and string conversion.

This commit adds support for parsing SCTP packets and converting them
to strings. Currently only chunks, parameters and error causes specified
in RFC 4960 are supported. Support for other RFCs will be added in the
future.
Tested on FreeBSD (amd64 and arm) and Linux.

Signed-off-by: Michael Tuexen <tuexen@fh-muenster.de>