pmbootstrap: fix listing of mounts if the file was deleted

Created by: zhuowei

On my system, /proc/mounts sometimes contains a line like

udev /media/zhuowei/redhd/pmbootstrap/chroot_native/dev/loop0p2\040(deleted)
 devtmpfs rw,relatime,size=1959476k,nr_inodes=489869,mode=755 0 0

The "\040(deleted)" text confuses pmbootstrap shutdown. Remove the suffix if we find it in a /proc/mounts entry.

This fixes #545 (closed).