[Dev] rename iceweasel user profile directory from 'firefox' to 'iceweasel'

[bill-auger]

this has been nagging me for some time - while fixing BR #3196,
i decided to address it finally

the rationale for this change is that, prior to this change,
if another 'firefox' is installed in addition to iceweasel,
both applications would share a profile, which is not very sane
behavior.

i added code to the PKGBUILD which should handle the change
gracefully and transparently to the user - i do not expect it to
cause any problems; but i wanted to get some opinions about it
- that code could be removed after a reasonable deprecation
period

it difficult to know why this was not done way back when;
but there is probably no technical reason to avoid doing so -
does anyone know of one?

i assume that the original rationale was that debian iceweasel
did not change it - however, debian probably did not change it,
because their iceweasel did not differ much from upstream (and
'iceweasel' was replacing their existing 'firefox') - our does
though; and it is only asking for problems, if another 'firefox'
is installed, especially if they are different versions

for example, currently, if a profile is created/used by a
firefox of vN, and later accessed by a firefox of vN-1, the
application will not start, unless the user elects to create a
new profile - i found a switch to disable that behavior; but it
makes more sense to use a dedicated profile for 'iceweasel', to
ensure that it is not used/shared with any other 'firefox'
variant

the original rationale may have also been to ease with migration
from arch - the change i added to the PKGBUILD handles that case
also (when `which firefox`, the profile is copied instead of
moved)