Add completions for mdadm

[PenegalECI]

Description

This PR adds completions for mdadm, the Linux tool for maintaining software RAID. This has been developed using the man page, so it should be rather exhaustive, taking into account around 98% of use cases AFAIK, but there could be some lacks due to:

• mdadm idiosyncrasies, like using the same short option for several long ones, or their different meanings in different contexts,
• a misunderstanding of the man, being only a regular mdadm user, not an expert,
• some specific combination of tags,
• remaining bugs or typos, although I checked my code along the whole coding.

TODOs:

• Changes to fish usage are reflected in user documentation/manpages.
• Tests have been added for regressions fixed
• User-visible changes noted in CHANGELOG.md

[faho]

No literal tabs, use \t.

The rest seems nice.

[faho]

Are these literal tab characters?

I'd prefer it if we used the \t escape instead.

It's even possible to use something like

printf '%s\t%s\n' "0" "Use original 0.90 format superblock" \
                          "0.90" "Use original 0.90 format superblock"
# and so on

[faho]

Or, a bit simpler:

echo "0"\t"Use original 0.90 format superblock"
echo "0.90"\t"..."

or countless variations. Anyway, please use \t, no tab characters in our files if we can help it.

[krader1961]

I agree with @faho. Do not include literal tabs (or other non-printable chars) in those strings since they are effectively invisible. Please use a symbolic representation like \t as @faho suggested so that it is obvious to anyone reading the script they are present.

[PenegalECI]

Here you are.

[faho]

And there you are - merged, thanks!