Enable multiline regexp mode for patterm matcher

[ihar-orca]

In the case of pattern comparison with the multiline string, an incorrect result may occur when the matched substring appears in the not first line.

Example:

#!/bin/bash

apache2_conf="/etc/httpd/conf/httpd.conf
/etc/apache2/httpd.conf"

if [[ "$apache2_conf" == */etc/apache2* ]]; then
  exit 0
else
  exit 1
fi

returns 1 because generated regexp ^.*/etc/apache2.*$ (interp/runner.go, func match()) is not multiline enabled.
this patch adds the (?m) flag into all autogenerated regexps in this function.

[ihar-orca]

Sorry for multiple commits, it seems that it's not squashed on my side

[mvdan]

SGTM but please add a test :)

[ihar-orca]

@mvdan test added

[mvdan]

Thank you!

[mvdan]

This branch cannot be rebased due to conflicts

Not sure how you ended up there. Can you try squashing and rebasing your commits?

[mvdan]

Ah, ignore me, I can squash-merge.