Curl_proxyCONNECT: remove superfluous statement

The variable is already assigned, skip the duplicate assignment.

Pointed out by cppcheck.
Curl_pp_readresp: remove superfluous assignment

Variable already assigned a few lines up.

Detected by cppcheck.
Curl_pp_flushsend: skip superfluous assignment

Detected by cppcheck.
Curl_add_timecondition: skip superfluous varible assignment

Detected by cppcheck.
code cleanup: we prefer 'CURLcode result'

... for the local variable name in functions holding the return
code. Using the same name universally makes code easier to read and
follow.

Also, unify code for checking for CURLcode errors with:

 if(result) or if(!result)

instead of

 if(result == CURLE_OK), if(CURLE_OK == result) or if(result != CURLE_OK)