Remove PT SOCKS argument length limit when SOCKS4 is used

Repeating from comment:31:ticket:3594:

After reading legacy/trac#7153 (moved) and looking at faf4f6c6, I see that in validate_transport_socks_arguments() I reject SOCKS arguments larger than MAX_SOCKS5_AUTH_SIZE_TOTAL even if I don't know the SOCKS version that the pluggable transport proxy is going to use. Maybe this check shouldn't happen in validate_transport_socks_arguments() so that we allow large SOCKS arguments if SOCKS4a is used.

Yawning Angel experienced this limit while coding his/her SSH pluggable transport: https://lists.torproject.org/pipermail/tor-dev/2013-June/005083.html

Unfortunately, this is not trivial to fix because when validate_transport_socks_arguments() is called we still don't know which SOCKS version the transport proxy supports.

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