Add support for linux rt schedule functions

[alxhill]

What does this PR do

Adds lightweight wrappers around setting and getting the scheduler and schedule parameters.

Related: #1260

Checklist:

• I have read CONTRIBUTING.md
• I have written necessary tests and rustdoc comments
• A change log has been added if this PR modifies nix's API

[alxhill]

Looks like the tier3 targets are failing due to a new clippy lint:

error: lifetime flowing from input to output with different syntax can be confusing
    --> src/sys/socket/sockopt.rs:1958:17
     |
1958 |     fn new(val: &OsString) -> SetOsString {
     |                 ^^^^^^^^^     ----------- the lifetime gets resolved as `'_`
     |                 |
     |                 this lifetime flows to the output
     |
help: one option is to remove the lifetime for references and use the anonymous lifetime for paths
     |
1958 |     fn new(val: &OsString) -> SetOsString<'_> {
     |                                          ++++

[alxhill]

@asomers are you the right person to request review for this PR? have been using it successfully in a private repo, seems to work as expected on PREEMPT_RT