chore(settings): refactor settings processing (#756)

- Better settings tree structure logged using `qdm12/gotree`
- Read settings from environment variables, then files, then secret files
- Settings methods to default them, merge them and override them
- `DNS_PLAINTEXT_ADDRESS` default changed to `127.0.0.1` to use DoT. Warning added if set to something else.
- `HTTPPROXY_LISTENING_ADDRESS` instead of `HTTPPROXY_PORT` (with retro-compatibility)

go.sum

@@ -115,8 +115,12 @@ github.com/qdm12/goshutdown v0.3.0 h1:pqBpJkdwlZlfTEx4QHtS8u8CXx6pG0fVo6S1N0MpSE
 github.com/qdm12/goshutdown v0.3.0/go.mod h1:EqZ46No00kCTZ5qzdd3qIzY6ayhMt24QI8Mh8LVQYmM=
 github.com/qdm12/gosplash v0.1.0 h1:Sfl+zIjFZFP7b0iqf2l5UkmEY97XBnaKkH3FNY6Gf7g=
 github.com/qdm12/gosplash v0.1.0/go.mod h1:+A3fWW4/rUeDXhY3ieBzwghKdnIPFJgD8K3qQkenJlw=
-github.com/qdm12/ss-server v0.3.0 h1:BfKv4OU6dYb2KcDMYpTc7LIuO2jB73g3JCzy988GrLI=
-github.com/qdm12/ss-server v0.3.0/go.mod h1:ug+nWfuzKw/h5fxL1B6e9/OhkVuWJX4i2V1Pf0pJU1o=
+github.com/qdm12/gotree v0.2.0 h1:+58ltxkNLUyHtATFereAcOjBVfY6ETqRex8XK90Fb/c=
+github.com/qdm12/gotree v0.2.0/go.mod h1:1SdFaqKZuI46U1apbXIf25pDMNnrPuYLEqMF/qL4lY4=
+github.com/qdm12/govalid v0.1.0 h1:UIFVmuaAg0Q+h0GeyfcFEZ5sQ5KJPvRQwycC1/cqDN8=
+github.com/qdm12/govalid v0.1.0/go.mod h1:CyS/OEQdOvunBgrtIsW93fjd4jBkwZPBjGSpxq3NwA4=
+github.com/qdm12/ss-server v0.4.0 h1:lMMYfDGc9P86Lyvd3+p8lK4hhgHUKDzjZC91FqJYkDU=
+github.com/qdm12/ss-server v0.4.0/go.mod h1:AY0p4huvPUPW+/CiWsJcDgT6sneDryk26VXSccPNCxY=
 github.com/qdm12/updated v0.0.0-20210603204757-205acfe6937e h1:4q+uFLawkaQRq3yARYLsjJPZd2wYwxn4g6G/5v0xW1g=
 github.com/qdm12/updated v0.0.0-20210603204757-205acfe6937e/go.mod h1:UvJRGkZ9XL3/D7e7JiTTVLm1F3Cymd3/gFpD6frEpBo=
 github.com/riobard/go-bloom v0.0.0-20200614022211-cdc8013cb5b3 h1:f/FNXud6gA3MNr8meMVVGxhp+QBTqY91tM8HjEuMjGg=