Ensure syntactically correct 'disk' and 'part' entries in disklayout.conf

[jsmeix]

• Type: Enhancement

• Impact: Normal

• Reference to related issue (URL):
  Broken 'part' entries in disklayout.conf in case of 'unknown' partition table #2801 (comment)

• How was this pull request tested?
  I got the same disklayout.conf on my homeoffice laptop
  with the changes of this pull request here.

• Brief description of the changes in this pull request:

In layout/save/GNU/Linux/200_partition_layout.sh
ensure syntactically correct 'disk' and 'part' entries

[jsmeix]

This pull request was triggered by
#2801 (comment)
therein in particular the last part that reads

I will have a look at
usr/share/rear/layout/save/GNU/Linux/200_partition_layout.sh
how to make it behave more reliably and failsafe,
in particular error out directly therein when things failed
instead of error out later in 950_verify_disklayout_file.sh

Therefore the changes in this pull request are
generic enhancements to detect early i.e. directly
in the code where the entries are genereated in
layout/save/GNU/Linux/200_partition_layout.sh
when invalid entries would be generated
(regardless what the actual reason is), see
#2801 (comment)

Because I get same disklayout.conf on my homeoffice laptop
there should be no regressions because of those changes
so that I would like to merge them tomorrow afternoon
unless there are objections form one of you
@rear/contributors

[jsmeix]

I will wait until @pcahyna had done his tests, see
#2803 (comment)

[pcahyna]

For tests with more diverse disk layouts see #2803

[jsmeix]

Via
0a1d634
we do no longer error out when there is no partition label type value
for a 'disk' entry in disklayout.conf because "rear recover" works
in a special case without partition label type value when there is
only a 'disk' entry but nothing else for this disk exists in disklayout.conf
which can happen when /dev/sdX is an empty SD card slot without medium, see
#2810