Pass `--enable=all` to shellcheck within contrib/

Pass `--enable=all` to shellcheck within contrib/

- Remove `SHELLCHECK_IGNORE` in favor of inline suppressions
  and more general `SHELLCHECK_OPTS`.

- Exclude `SC2250` (turned on by `--enable=all`) globally

- Pass `--enable=all` to shellcheck for scripts in contrib/: it's
  very important to catch errors early in areas that are not easily
  testable.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: szubersk <szuberskidamian@gmail.com>
Closes #12760