retitle PRIVATE tabs when other person /nicks

Though we'll only see the NICK if we happen to share a channel with them, it's
 a common enough occurrence that it's handy
/msg: use makeptab, rather than duplicating it (and adding mistakes)
Refactor all the bufs[bufs[cbuf].server] to use new macro SERVER(buf)
Use SERVER macro for b2 as well, not just cbuf
Generate numeric.h from a master data list (in Python)

This is preparatory to adding conversion back to names
Give names for recognised but unhandled numerics
irc.c: Fix handling of ERROR with message