Add users/groups; Env changes; Undo Rayon; Docs

A username and group name can optionally be included in a service config
file so that that service is run as a user other than root. These can be
defined on the service level (applies to every method), or
defined/overridden on the method level.

I also wrote up some documentation here, hopefully can help people
approaching the codebase for the first time.
2 jobs for dependencies in 2 minutes and 20 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build
failed #8316
allowed to fail
build:linux:cargo

00:01:44

passed #8317
build:redox:cargo

00:02:20

 
Name Stage Failure
failed
build:linux:cargo Build

error: aborting due to 6 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=1dc22dfe3f2b0baa -C extra-filename=-1dc22dfe3f2b0baa --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-13ad813b32f5c402.rlib --extern failure=/builds/redox-os/init/target/debug/deps/libfailure-9492293dc951d670.rlib --extern generational_arena=/builds/redox-os/init/target/debug/deps/libgenerational_arena-0d7dcc537bc8aff3.rlib --extern log=/builds/redox-os/init/target/debug/deps/liblog-8c8fe18807701d46.rlib --extern syscall=/builds/redox-os/init/target/debug/deps/libsyscall-5371cfcbc0e22a3e.rlib --extern redox_users=/builds/redox-os/init/target/debug/deps/libredox_users-dcd4206845df3ece.rlib --extern serde=/builds/redox-os/init/target/debug/deps/libserde-29c47034ac5063ac.rlib --extern serde_derive=/builds/redox-os/init/target/debug/deps/libserde_derive-cc774d49f25c9d59.so --extern simple_logger=/builds/redox-os/init/target/debug/deps/libsimple_logger-6e91b932e2029743.rlib --extern toml=/builds/redox-os/init/target/debug/deps/libtoml-bc9747a1ec29e59d.rlib -L native=/builds/redox-os/init/target/debug/build/backtrace-sys-82bf9e08fd741eb7/out` (exit code: 1)
ERROR: Job failed: exit code 1