Fix some minor issues identified by shellcheck

Fixes #23 @ github
Use UUIDs in /boot/grub/grub.cfg for VM builds rather than hardcoding /dev/sda

This allows booting the image in KVM as either sda or vda without
requiring any manual changes in /boot/grub/grub.cfg.  Use
FIXED_DISK_IDENTIFIERS="yes" for VM builds by default.

Patch rebased by Michael Prokop <mika@grml.org>
Fixes #29 @ github
config: use uuidgen tool for DISK_IDENTIFIER usage example

Fixes #29 @ github
Fix some further issues identified by shellcheck

Fixes #23 @ github
Do not use fixed disk identifiers by default

This is calling for problems when dealing with multiple
disks/installations at the same time that have been installed via
grml-debootstrap.

Fixes #29 @ github
Always use UUIDs for GRUB booting in VMs instead of hardcoding values

Only necessary for wheezy and older Debian releases, since jessie
it's working fine ootb.
docs: fix wrong usage example description

Thanks: Florian Klien for the bug report
Execute kpartx in sync mode to avoid race conditions

Otherwise the according loop device might not be present yet
when we're trying to access it (via mkfs).
Support GRUB as present in Debian/jessie in VM builds

Fixes #30 @ github
docs: set man.base.url.for.relative.links option when generating man-page

This avoids the strange:

| 1. [set $man.base.url.for.relative.links]/images/screenshot.png

message in the generated man-page.
Fix linux-image package check for Debian/jessie 32bit

The linux-image-686 package no longer exists in jessie,
instead we have to use linux-image-686-pae nowadays.

Thanks: Patrick Schleizer <adrelanos@riseup.net> for the bug report
Fixes #32 @ github
Fix check for available /dev/md* devices

/dev/md* might also expand devices like /dev/md/0:

| # ls -la /dev/md*
| brw-rw---- 1 root disk 9, 0 Oct 13 19:01 /dev/md0
|
| /dev/md:
| total 0
| drwxr-xr-x  2 root root   60 Oct 13 19:01 ./
| drwxr-xr-x 15 root root 3860 Oct 13 19:02 ../
| lrwxrwxrwx  1 root root    6 Oct 13 19:01 0 -> ../md0

We're interested only in /dev/md[0-9]* devices though.
Release new version 0.67