httpcaddyfile: Fix default issuers when email provided
If `tls <email>` is used, we should apply that to all applicable default issuers, not drop them. This refactoring applies implicit ACME issuer settings from the tls directive to all default ACME issuers, like ZeroSSL.
We also consolidate some annoying logic and improve config validity checks.
Ref: https://caddy.community/t/error-obtaining-certificate-after-caddy-restart/11335/8
ap.Issuers=[]certmagic.Issuer{acme,zerossl}// TODO: keep this in sync with Caddy's other issuer defaults elsewhere, like in caddytls/automation.go (DefaultIssuers).
// if a non-ZeroSSL endpoint is specified, we assume we can't use the ZeroSSL issuer successfully
// if a specific endpoint is configured, can't use multiple default issuers