for gcc >= 2.5 and no-ops for gcc >= 2.6. Converted to use __dead2
or __pure2 where it wasn't already done, except in math.h where use
of __pure was mostly wrong.
preen (-p), and in that case the filesystem is skipped if it is clean.
A new flag "-f" for 'force' has been added which basically gives back
the old behavior of checking all the filesystems all the time. This
very closely models the behavior of SunOS and Ultrix.
