Add 'disksup_posix_only' parameter to disksup

On embedded (Linux) systems the "df" program is very often provided by
the Busybox (or Toolbox) project. This version does not provide the "-l"
option (for displaying only local filesystems) used in disksup.erl.

To make disksup work on these embedded platforms, this patch adds a new
application parameter 'disksup_posix_only', to make diskup use only
options defined in the POSIX standard:
http://pubs.opengroup.org/onlinepubs/9699919799/utilities/df.html