Warn when functions that exit are invoked in subshells

[koalaman]

For new checks and feature suggestions

• shellcheck.net (i.e. the latest commit) currently gives no useful warnings about this
• I searched through https://github.com/koalaman/shellcheck/issues and didn't find anything related

Here's a snippet or screenshot that shows the problem:

Inspired by #1158

#!/bin/sh

my_mktemp() {
  mktemp || exit 42
}

dir=$(my_mktemp)
echo "$dir"

Here's what shellcheck currently says:

Nothing

Here's what I wanted or expected to see:

The exit is buggy here, since the function is only ever called in a subshell so it'll never do anything other than return