Use setenv in qputenv if possible, since it won't leak

Use setenv in qputenv if possible, since it won't leak

putenv(3) is evil: SUSv2 requires that the pointer passed to it be added
to the environment and that modifying the contents of that pointer later
will also cause the environment to change. That means we needed to
strdup before calling it and that memory was never freed.

This shows up all the time in valgrind's leak check.

Instead, let's use the 4.3BSD & POSIX.1-2001 setenv(3) function, which
does copy. That means there are either no leaks or, if there are,
they're not our fault.

Change-Id: I4576f91cc718b6b3cae790c4f2854c4976dded37
Reviewed-by: Qt Doc Bot <qt_docbot@qt-project.org>
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>