Support Namespaces; Use syscalls to impl Command

While attempting to implement namespaces for services, I found that
std::process::Command was inadequate for the task, so this commit
includes a reimplementation of it with redox syscalls. It's not as good,
but it does what I need, and hopefully will be improved upon as I
continue to work on this.

Namespaces work now!
2 jobs for dependencies in 2 minutes and 22 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
failed #8389
allowed to fail
build:linux:cargo

00:01:37

passed #8390
build:redox:cargo

00:02:22

 
Name Stage Failure
failed
build:linux:cargo Build

error: aborting due to 7 previous errors

For more information about this error, try `rustc --explain E0308`.
error: Could not compile `init`.

Caused by:
process didn't exit successfully: `rustc --edition=2018 --crate-name init src/main.rs --color never --crate-type bin --emit=dep-info,link -C debuginfo=2 -C metadata=8df5c6f996d26cb4 -C extra-filename=-8df5c6f996d26cb4 --out-dir /builds/redox-os/init/target/debug/deps -C incremental=/builds/redox-os/init/target/debug/incremental -L dependency=/builds/redox-os/init/target/debug/deps --extern chashmap=/builds/redox-os/init/target/debug/deps/libchashmap-69bf9977fda223f6.rlib --extern failure=/builds/redox-os/init/target/debug/deps/libfailure-df8afc23349ddfce.rlib --extern generational_arena=/builds/redox-os/init/target/debug/deps/libgenerational_arena-bb0515714de2bf83.rlib --extern log=/builds/redox-os/init/target/debug/deps/liblog-10a7b1eb5887bb76.rlib --extern syscall=/builds/redox-os/init/target/debug/deps/libsyscall-81151f40a54fb451.rlib --extern redox_users=/builds/redox-os/init/target/debug/deps/libredox_users-9c10f03b9a72b152.rlib --extern serde=/builds/redox-os/init/target/debug/deps/libserde-946c3560de3894f4.rlib --extern serde_derive=/builds/redox-os/init/target/debug/deps/libserde_derive-6db6c18e19239172.so --extern simple_logger=/builds/redox-os/init/target/debug/deps/libsimple_logger-2fd7bd92014e39a7.rlib --extern toml=/builds/redox-os/init/target/debug/deps/libtoml-215cbdd329156cb7.rlib -L native=/builds/redox-os/init/target/debug/build/backtrace-sys-8af89e5432d3a0fb/out` (exit code: 1)
ERROR: Job failed: exit code 1