Fixed typo.

We can do without those on the install media.  Instead, route
necessary code parts through is_install_media().  A lot of
stale and forgotten code got ditched in the process--e.g. when
did we ever run without an /etc/fstab?

Platforms are all the same, except when / is not writable.  That
only ever matters once, when we fake it writeable using unionfs.

Fear not as we will reintroduce the functionality in the GUI using
our new firmware update system which includes pkg(8)/opnsense-update(8).

* Remove spurious debug output that was never enabled and make
  actual error messages the default behaviour.

* Update links and remove a bit of softcoding that was going
  on around the GUI through global variables (we don't want to
  hide the help menu, nope).

We have moved on to other means of firmware upgrades.  Gives us
the opportunity to do a little spring clean here.  (Plus I don't
have to fix the gzsig port anymore.)