This commit adds stubs for the `ExitError` struct of the `exec` package
and `ProcessState.ExitCode()` of the `os` package.
Since the `os/exec` is listed as unsupported, stubbing these methods
and structs should be enough to get programs that use these to compile.
This is necessary for the following:
- to make sure os/exec can be imported
- to make sure internal/testenv can be imported
The internal/testenv package (which imports os/exec) is used by a lot of
tests. By adding support for it, more tests can be run.
This commit adds a bunch of new packages that now pass all tests.
This commit implements various process related functions like
os.Getuid() and os.Getpid(). It also implements or improves this support
in the syscall package if it isn't available yet.
