[PATCH 0/2 V2] fix up mount check handling

[Eric Sandeen]

A 2 patch series to more cleanly handle the disappearance
of ustat() on some platforms, used in platform_check_ismounted()
on Linux, which caused a switch to getmntent loops in _ismounted()

This series collapses the now-similar check_platform_ismounted()
and platform_check_iswritable() functions into one, with
helpers.

Thanks,
-Eric