installer: Fix handling of CMOS set to UTC vs. CMOS set to wall time.

installer: Fix handling of CMOS set to UTC vs. CMOS set to wall time.

It was reversed in the installer (compared to tzsetup(8)).

/etc/wall_cmos_clock needs to be created when the CMOS is set to the
time of the clock on the wall. While here, also add removing the file
in case the user selects 'CMOS set to UTC'. This is if he chose the
other setting previously and then tried again.

Reported-by:   many
Dragonfly-bug: <http://bugs.dragonflybsd.org/issues/39>
Dragonfly-bug: <http://bugs.dragonflybsd.org/issues/2060>