runcon: added implementation and tests.

[koutheir]

Added implementation and tests for the runcon command.

For reference: Supporting-SELinux-in-the-coreutils.

The following testing tasks are still pending for this command:

• Check expected exit statuses of the command.
• Check the implementation of --compute somehow.
  I checked this manually by comparing the operation of the GNU implementation with our implementation through strace. I hope we can find a way to check this automatically instead.
• Use unified errors (uucore/error.rs).

All runcon tests succeed on a virtual machine with SELinux enabled:

$ cargo test --features=feat_selinux -- test_runcon::
    Finished test [unoptimized + debuginfo] target(s) in 0.31s
     Running unittests (target/debug/deps/coreutils-2df32bef2901c472)

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

     Running tests/test_util_name.rs (target/debug/deps/test_util_name-3c1c205336515b79)

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 23 filtered out; finished in 0.00s

     Running tests/tests.rs (target/debug/deps/tests-ba1796e2d5b70410)

running 6 tests
test test_runcon::help ... ok
test test_runcon::plain_context ... ok
test test_runcon::version ... ok
test test_runcon::print ... ok
test test_runcon::invalid ... ok
test test_runcon::custom_context ... ok

test result: ok. 6 passed; 0 failed; 0 ignored; 0 measured; 1332 filtered out; finished in 0.12s

Closes #1973.

[sylvestre]

You are really impressive! Only stty will be missing after this is merged!

[miDeb]

Thank you for your work! I only found some small issues.

[koutheir]

@miDeb, thank you for the review. I implemented the discussed changes. Can you take a look?

[koutheir]

The following testing tasks are still pending for this command:

• Check expected exit statuses of the command.
• Check the implementation of --compute somehow.
  I checked this manually by comparing the operation of the GNU implementation with our implementation through strace. I hope we can find a way to check this automatically instead.
• Use unified errors (uucore/error.rs).

[miDeb]

Check expected exit statuses of the command.

You can do this with the code_is() function in tests. E.g. new_ucmd!().args(args).fails().code_is(126).

[sylvestre]

Only stty left as a program, wahou!

[koutheir]

Only stty left as a program, wahou!

stty (at least, the GNU implementation thereof) is an ugly mess...