Fix off-by-one.

MFC after:	1 day
Check buffer length more strictly.

MFC after:	1 day
Add ${name}_env and ${name}_prepend.  ${name}_env is an argument list which
will be passed to env(1).  ${name}_prepend is simply prepended to the command
line for $command.
Add env and prepend to _rc_namevarlist.
Merge remote-tracking branch 'freebsd/master' into hardened/current/master