Fix shellcheck version >= 0.5.0 errors

Summary:
The linter will throw warnings and errors with shellcheck versions >=
0.5.

Backport of core PR16327 (which implies PR14115, PR15164 and PR15166)
https://github.com/bitcoin/bitcoin/pull/16327/files

https://github.com/bitcoin/bitcoin/pull/14115/files
https://github.com/bitcoin/bitcoin/pull/15164/files
https://github.com/bitcoin/bitcoin/pull/15166/files

Note that the rule SC1117 (from PR14115) is still excluded as it would
throw false positives. It has been removed from shellcheck versions >
0.5.0, but I kept it for compatibility with previous versions (i.e.
Debian buster has 0.5.0).
See https://github.com/koalaman/shellcheck/wiki/SC1117 for details.

Test Plan:
Install the latest shellcheck (currently 0.7.0).
Check arc lint --everything returns no error.
Tested with versions 0.4.4, 0.4.6, 0.5.0 and 0.7.0.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3969