Move the ip_fragment code from ip_output() to a separate function,
so that it can be reused elsewhere (there is a number of places
where it can be useful). This also trims some 200 lines from
the body of ip_output(), which helps readability a bit.

(This change was discussed a few weeks ago on the mailing lists,
Julian agreed, silence from others. It is not a functional change,
so i expect it to be ok to commit it now but i am happy to back it
out if there are objections).

While at it, fix some function headers and replace m_copy() with
m_copypacket() where applicable.

MFC after: 1 week