reject: Remember and send reasons for rejections

reject: Remember and send reasons for rejections

rejectcache entries can now use either a K-line aconf or a static
string as a reason. This will be sent in a 465 numeric before the usual
ERROR. In the case of K-lines, it resembles the 465 you would have been
sent without being rejected:

; nc -s 127.6.6.6 127.0.0.1 5000
:staberinde.local 465 * :You are banned from this server- Temporary
    K-line 4320 min. - abc123 (2019/12/31 01.07)
ERROR :Closing Link: (*** Banned (cache))
; nc -s 127.128.0.0 127.0.0.1 5000
:staberinde.local 465 * :You are not authorised to use this server.
ERROR :Closing Link: (*** Banned (cache))