Fish builds and runs! The patch is refreshingly simple:
* mount.h is disabled because we don't have it.
* A recent revision of libc is needed for Redox constants. Types changed
a bit between releases, so some casts had to be removed or added.
* We don't have pselect, so I had to hack around it by mimicking how it
works.
Unfortunately, running programs or commands hangs unless part of a
pipeline. So, `ls` hangs but `ls &` works. It seems like a waitpid issue
rather a fork/exec issue.