getinfo: add CURLINFO_POSTTRANSFER_TIME_T

Returns the time, in microseconds, from the start until the last byte is
sent by libcurl (i.e. the request is sent off).

Closes #14189

test 1541: verify getinfo values on first header callback

Reported-by: chensong1211 on github
Ref: #13125
Closes #13128