Bash completions: warn and exit if POSIX mode detected

[maxim-belkin]

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Bash completions: test if POSIX mode is enabled, print an error message and exit out.
Fixes #14643

The check itself is written in POSIX-compliant manner.
The message is written to standard error.
I think this check shouldn't return a non-zero exit code, but I'm open to suggestions / opinions.

[BrewTestBot]

Review period will end on 2023-02-17 at 16:15:37 UTC.

[BrewTestBot]

Review period skipped due to critical label.

[MikeMcQuaid]

Makes sense to me, thanks again @maxim-belkin!