sh: remove dependencies

[grisenti]

this PR removes the nix and atty dependencies from sh

[jgarzik]

looks like mac CI has a few nits:

error[E0425]: cannot find function `__errno_location` in crate `libc`
   --> sh/os/errno.rs:409:33
    |
409 |     let errno = unsafe { *libc::__errno_location() };
    |                                 ^^^^^^^^^^^^^^^^ not found in `libc`

error[E0560]: struct `sigaction` has no field named `sa_restorer`
   --> sh/os/signals.rs:281:9
    |
281 |         sa_restorer: None,
    |         ^^^^^^^^^^^ `sigaction` does not have this field
    |
    = note: all struct fields are already assigned

looks great otherwise

[grisenti]

@jgarzik should be fixed now