- Added constant sizes for usernames and the nonce in citadel.h

- Added constant sizes for usernames and the nonce in citadel.h
- The server now seeds its PRNG with the microsecond time at startup.  The
  random numbers are needed for the APOP auth scheme's nonce
- A nonce is displayed at the banner, conforming to the APOP RFCs
- Added MD5 (APOP) auth via a module PAS2 (password scheme 2)
  This will only work for "real" bbs accounts
- Added APOP command to serv_pop3
- Reorganized a couple of user commands