ftpd: Parse commands in RFC 2228.

Let the parser recognize these security
extensions, but mark them as unimplemented.
ftp: Detect one-time passwords.

Follow RFC 2228 when reacting to a
challange-response password request.
ifconfig: Convenience format named `check'.
Further tests of ifconfig.
ifconfig: Correction to flag listing.
Corner test case for ifconfig.

GNU/Linux displays problems while bringing up the
interface after an address change.
ifconfig: No listing of unchangeable flags.

Many interface flags are known to be read-only.
Avoid mentioning them in the help message.
ifconfig: Allow flags richer than 16 bits.

Contemporary systems are not restricted to flags within
a 16 bit span, notably FreeBSD and GNU/kFreeBSD.  Support
this wider range and incorporate three new flags.