case-insensitive base32 decode and customizable max sub-domain length

[patterniha]

Currently, most of DNS IPs do not work in Iran, according to tests, this is for two reasons:

GFW changes the subdomain capitalization:
Randomly capitalizes some letters and lowercases others in request subdomain.
for example: abcdefgh.example.com is converted to aBcDEfgH.example.com
It doesn't affect normal dns requests to big dns providers like Cloudflare, because Cloudflare is not case sensitive, so it only affects slipstream.

GFW limit subdomain length:
for example according to @FDeghy tests:

OXNWZGDTUGNGFBUALMDSYTLGAU6B5Z4XPJBXWUJHIYBBWL6MNZSVLMVEX.DYYJQ2LODHWG3JIBKHBTLLYVPP.abc.aaaaaaaaaa.ir (102 total length)

doesn't reach the server, but

OXNWZGDTUGNGFBUALMDSYTLGAU6B5Z4XPJBXWUJHIYBBWL6MNZSVLMVEX.DYYJQ2LODHWG3JIBKHBTLLYVP.abc.aaaaaaaaaa.ir (101 total length)

reaches the server.

so we need a option to control max total subdomains length.

///

These are urgent needs for Iranians. These are happening to most of the DNS Ips, and right now the only way to access the internet is DNS-tunnel.

[patterniha]

and for the second problem: Mygod/slipstream-rust#3

[patterniha]

according to new tests, we don't have a length-limit, we have rate-limit ! No, we still have length limit.

[patterniha]

unfortunately, length limit still exists, most DNS allow only maximum of 101 domain length.

also, it seems that we have no rate limit when source ports of requests are different.