pkg/58264: Enable FAILOVER_FETCH by default

[rwhitlock22%gmail.com@localhost]

>Number:         58264
>Category:       pkg
>Synopsis:       Enable FAILOVER_FETCH by default
>Confidential:   no
>Severity:       non-critical
>Priority:       medium
>Responsible:    pkg-manager
>State:          open
>Class:          change-request
>Submitter-Id:   net
>Arrival-Date:   Thu May 16 23:20:00 +0000 2024
>Originator:     Robert Whitlock
>Release:        pkgsrc-2024Q1
>Organization:
>Environment:
>Description:
This is a request to enable FAILOVER_FETCH by default. Doing so would check that the right distfiles are actually downloaded, which seems to be a good idea to avoid weird mysterious problems as well as upstream switcheroo shenanigans. Wanting the checksum to not be checked seems like it would be the exception rather than the rule, and most likely the user would know he would want to ignore the checksum in those situations. The user could disable FAILOVER_FETCH in those situations.
>How-To-Repeat:
Read pkgsrc/mk/fetch/fetch.mk and discover that FAILOVER_FETCH needs to be defined for checksums to be checked.
>Fix: