code cleanups and general performance work to be developed in relative peace.
While cleaning up the networking code /is/ important, fixing QF's perfomance
issues is of much higher priority.
net_svc.c, and add add a NET_SVC_GetString function to access it.
As an added bonus, it actually boundschecks it!
- preexpand that annoying SHOWNET macro, which was only used twice,
and was quite small and pointless
- whitespace cleanup and a comment typo fix
- converted svc_packetentities and svc_deltapacketentities
- added some convenience defines for U_foo grouping
- tweaked Host_NetError's "please report this" print, but it still
doesn't work
