socks: Fix blocking timeout logic

- Document in Curl_timeleft's comment block that returning 0 signals no
  timeout (ie there's infinite time left).

- Fix SOCKS' Curl_blockread_all for the case when no timeout was set.

Prior to this change if the timeout had a value of 0 and that was passed
to SOCKET_READABLE it would return right away instead of blocking. That
was likely because it was not well understood that when Curl_timeleft
returns 0 it is not a timeout of 0 ms but actually means no timeout.

Ref: https://github.com/curl/curl/pull/5214#issuecomment-612512360

Closes https://github.com/curl/curl/pull/5220

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