FEAT: check if using dot command in posix shell without a slash in the filename

[jumanjiman]

For new checks and feature suggestions

• [x] shellcheck.net (i.e. the latest commit) currently gives no useful warnings about this
• [x] 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:

#!/bin/sh
. path_without_a_slash

Here's what shellcheck currently says:

SC1091: Not following: path_without_a_slash was not specified as input (see shellcheck -x).

Here's what I wanted or expected to see:

SC1091: Not following: path_without_a_slash was not specified as input (see shellcheck -x). SCxxxx: In POSIX sh, dot command follows PATH if filename lacks a slash

POSIX specification

http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#dot

[pabs3]

Please note that this syntax works in bash, because it adds the current directory to the path used for the dot command.