Fix wxExecute() default priority if wxProcess is not used.

The code mistakenly assigned the lowest possible priority (0, in wx's
numbering), while the intention probably was to have the same default as
wxProcess, which is 0 in POSIX numbering and 50== wxPRIORITY_DEFAULT in
wx's.

Fixes permission denied errors if lowering priority is not permitted.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74956 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Workaround OS X crash when closing FDs in wxExecute.

wxExecute() closes all file descriptors in the forked child process.
This is a common practice, but it unfortunately breaks in combination
with libdispatch on OS X, which - instead of gracefully handling the
situation - intentionally(!) crashes when something closes its kevent
file descriptor in OS X 10.8.

There's doesn't seem to be a simple way to get this descriptor and
there's no telling when libdispatch is pulled into the executable, so
just leave the file descriptors be when running under Darwin, until a
better fix is found.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74957 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775