do not use fileno calls in forked child

This fixes an issue that causes QProcess::start to silently fail on
OS X 10.9. Apparently, fileno(stdout) locks the handle on OS X 10.9.
It may happen that the parent process fflush()s stdout while the child
has just been forked. The stdout lock of the parent is never released
in the child and fileno(stdout) therefore locks forever.

According to the fork documentation on opengroup.org one may only call
async-signal-safe functions between fork and exec in the child. The
fileno() function does not appear in the list of async-signal-safe
functions. Also, fileno(stdout) and friends can be easily replaced by
the standard constants STDOUT_FILENO etc.

Done-with: Fawzi Mohamed <fawzi.mohamed@digia.com>
Task-number: QTBUG-37306

Change-Id: I2b1f5f47cc48a1ad020fb0493a955d2bc27aeb47
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from b2216bbe06b8be2bef6d8bc2ffff1337f6d23358)