at mycroft's suggestion, i've added a new section to the
manual pages - ``SECURITY CONSIDERATIONS''.

i've gone through a fair few of the manual pages (all those with
BUGS or CAVEAT sections) and moved any items that were more
relevant to the new category to the latter.

this is just a start, however. if you feel that there's more
that could be added to various manual pages that fit this category,
please submit a PR to the `security' category, `doc-bug' class.

luke.