Update config/env.c to accept conventional environment variables in a…

[iain-henderson]

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What does this PR aim to accomplish?:

Update environment variable parsing to allow conventional environment variables.
Both FTLCONF_dns_upstreams and FTLCONF_DNS_UPSTREAMS should configure dns.upstreams

How does this PR accomplish the above?:

The parsing code is updated to also check against an uppercase version of the environment key.

Link documentation PRs if any are needed to support this PR:

pi-hole/docs#1180

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