Filesystem: Support symlink as mountpoint directory

Filesystem monitor operation fails when the directory attribute is a
symlink.

The monitor operation calls the list_mounts function, which cats
/proc/mounts if it exists, else cats /etc/mtab if it exists, else
runs the mount command. It then greps for " $MOUNTPOINT " in the
output, where $MOUNTPOINT is the value of the directory attribute.

/proc/mounts, /etc/mtab, and the mount command resolve symlinks
to their canonical targets. So while the monitor operation greps for
the symlink path (surrounded by spaces) as defined in the directory
attribute, the symlink will not be present in the list_mounts output.
Only the symlink's target will be present.

This patch uses readlink -f $MOUNTPOINT to resolve the symlink to its
canonical name before using it as a grep pattern in the
Filesystem_status function.