Check for validity of values passed in through PIHOLE_DNS_

[PromoFaux]

Description

Adds a check using the already sourced valid_ip and valid_ip6 functions from basic-install.sh to ensure that any duff values passed in through PIHOLE_DNS_ are junked

Motivation and Context

See discussion here: #838

How Has This Been Tested?

Built it, tried it:

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[PromoFaux]

[kylekurz]

LGTM!