osproc: MacOSX workaround for lack of execvpe

MacOS X is the only Posix system (supported by Nimrod) that doesn't have execvpe. This code works on Linux, but I don't have any Mac to test on.

If you read this and happen to use MacOS, please test this patch with the following code:
```
import osproc, strtabs
var t: PStringTable = newStringTable({"foo": "bar"})
let i = startProcess("/usr/bin/env", options={poParentStreams}, env=t)
let code = i.waitForExit()
echo code
```
should return:
```
foo=bar
0
```