prevents the ftp bounce attack, and we should be secure out of the
box, not require users to tweak obscure stuff.
* allow the version string reported to clients to be changed with '-V vers'.
if vers is empty or `-', don't report a version.
* if -r is given, permanently drop root privs
* if not a REAL user (i.e, GUEST or CHROOT), and ftpd is running on a port
> IPPORT_RESERVED+1, permanently drop root privs
* don't bother reverting to root privs to logout of wtmp/utmp; since the
file descriptor is already open this isn't necessary.
* fix the binding of the port for the PORT/LPRT/EPRT connection to be the
ctrl_addr.su_port-1, not hardcoded to `20' (this was broken in the ipv6
merge). if root privs have been dropped, and this would be a port <
IPPORT_RESERVED, use a random port instead (which isn't RFC959 compliant
but it doesn't appear that many clients care).
* prevent login of a new user if privs have been dropped and already logged
in as a REAL user (existing check already stops GUEST & CHROOT users).
* move the port check stuff into a separate port_check() function, and use
for PORT, LPRT, and EPRT checks. inspired by freebsd
* minor KNF
* minor man page cleanup
d85f3bb159