o De-staticise things that don't need to be static.

o De-staticise things that don't need to be static.
o Bring the static ``ttystate'' into struct prompt so that
  the tilde context is per prompt and not global.
o Comment the remaining static variables so that it's
  clear why they're static.
o Add some XXX comments suggesting that our interface list
  and our hostname should be re-generated after a signal
  (say SIGUSR1) so that a machine with PCCARDs has a chance.