Cygwin: pty: Revise the code for timeout in term_has_pcon_cap().

- Sometimes timeout period in term_has_pcon_cap() may not be enough
  when the machine slows down for some reason. This patch eases the
  issue. In the new code, effective timeout period is expected to be
  extended as a result due to slowing-down the wait loop as well when
  the machine gets into busy.