Correct the function signatures for getpriority/setpriority

[chriswailes]

The __priority_which_t type was incorrectly exposed as a type alias and defined as unsigned when they should be signed.

Description

This CL removes the __priority_which_t type alias from the unix::linux_like::linux::gnu module. It is not used for the same purpose as in the C++ source (enum value definition) and it is not supposed to be exposed to the user (see man setpriority). The type signatures for getpriority and setpriority are then updated to use the correct c_int type.

Sources

Libc source references:
*https://github.com/bminor/glibc/blob/4052d99ead880797cf271309fd87ddd2b95bd353/resource/sys/resource.h#L105

• https://github.com/bminor/glibc/blob/4052d99ead880797cf271309fd87ddd2b95bd353/sysdeps/unix/sysv/linux/bits/resource.h#L187

C++ Standard Reference:

• Default underlying type for an enum is int: https://en.cppreference.com/w/cpp/language/enum

Checklist

• Relevant tests in libc-test/semver have been updated
• No placeholder or unstable values like *LAST or *MAX are
  included (see rust-lang/libc#3131)
• Tested locally (cd libc-test && cargo test --target mytarget);
  especially relevant for platforms that may not be checked in CI

Compilation error when running cargo test --target x86_64-unknown-linux-gnu:

error[E0061]: this function takes 3 arguments but 2 arguments were supplied
    --> ctest/src/lib.rs:2496:25
     |
2496 |                 Rc::new(macro_rules::compile(self.parse_sess, item)),
     |                         ^^^^^^^^^^^^^^^^^^^^                  ---- argument #2 of type `&RefCell<Features>` is missing
     |
note: function defined here
    --> /usr/local/google/home/chriswailes/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/garando_syntax-0.1.0/src/ext/tt/macro_rules.rs:172:8
     |
172  | pub fn compile(sess: &ParseSess, features: &RefCell<Features>, def: &ast::Item) -> SyntaxExtension {
     |        ^^^^^^^
help: provide the argument
     |
2496 |                 Rc::new(macro_rules::compile(self.parse_sess, /* &RefCell<Features> */, item)),
     |

This does not appear to be caused by this PR.

[chriswailes]

I apparently looked at the wrong portion of libc-test/semvar. I'm attempting to fix this now.