add comment for getargbool()

I always forget how getargbool works. Add a comment documenting it.
dracut-lib: add find_mount, use it to implement ismounted

find_mount is really the same thing as ismounted with two additions:

  1) uses "readlink" so "ismounted /dev/disk/by-label/LABEL" works
  2) returns the mountpoint of the device

And ismounted is now just "find_mount $dev >/dev/null".
dracut-functions: $_x should exist, no need to test.

Signed-off-by: Chao Wang <chaowang@redhat.com>
dracut.sh: test if we can lazy resolve with ldd
plymouth/plymouth-newroot.sh: fixed pre-pivot hook
dracut.spec: add Requires: kbd kbd-misc
set DRACUT_SYSTEMD for systemd mode in the initramfs
kernel-modules/module-setup.sh: add missing "fi"
dracut-functions.sh: inst_hook() with [0-9]-<name>
i18n/parse-i18n.sh: remove udev rules
base/dracut-lib.sh: fixed ismounted(<mountpoint>)
systemd/module-setup.sh: install systemd udev rules
virtfs/mount-virtfs.sh: don't exit != 0
plymouth/plymouth-newroot.sh: don't "exit 0" for sourced scripts
plymouth/plymouth-pretrigger.sh: don't "exit 0" for a sourced script
crypt: add systemd crypt support
crypt/crypt-run-generator.sh: do not add already existing luks
dmraid,mdraid,lvm,udev: don't process DM_MULTIPATH_DEVICE_PATH devs
TODO: update
TEST-10-RAID/test.sh: comment debug options