This is a NET/2 BSD su command, ported back to SunOS 4.x. Tested with
SunOS 4.1.3 and FreeBSD. Unlike System V su commands, this one resets
$HOME (so shell dotfiles should work) and will only let wheel members
su to root when the wheel group exists.

Optional feature: when build with -DSECURETTY, will accept the "root"
password only when the terminal is marked as secure (BSD) or when it is
the system console (SYSV).
