Add an 'enable' and 'disable' target to rcrun, and add softlink shortcuts

Add an 'enable' and 'disable' target to rcrun, and add softlink shortcuts
for 'rcenable' and 'rcdisable'.  These functions will modify rcrun-generated
entries in /etc/rc.conf.  They will not delete any manual entries in rc.conf
even if the manual entry is being overriden.

Add additional states to the varsym vars and make rc.subr and rcrun aware
of them.  The states are now:

    running failed disabled irrelevant configured stopped