Reschedule listening socket callback after socket errors

Any explicit _sock_cb() return, e.g. after a failed accept() or getpeername()
call leads to an unresponsive dbmail process as _sock_cb() is never
re-scheduled back.

getpeername() is easy to fail with nmap TCP scan:
> sudo nmap -sT -p143,993 192.168.197.140

Which immediately stalls the dbmail:
> Oct 24 07:37:45 etsys dbmail/imap4d[23523]: Info:[server] _sock_cb(+580):
> getpeername::error [Transport endpoint is not connected]
> Oct 24 07:37:45 etsys dbmail/imap4d[23523]: Info:[server] _sock_cb(+580):
> getpeername::error [Transport endpoint is not connected]