cleanup: add many compiler warning options

This is a set of warning options which are useful and IMO should be
enabled in secnet.  Currently the code is not warning-clean to the
required standard; subsequent changes will fix the warnings.

Signed-off-by: Richard Kettlewell <richard@greenend.org.uk>
Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>