Document the sys::wait module

[vkkoskie]

Noteworthy secondary changes included as a result of the primary documentation:

• Additions to unistd::Pid to support waitpid calling conventions
• Change the waitpid function signature (backward compatible)
• Application of rustfmt to unistd.rs and sys::wait.rs

Continued from work by

• Marcin Mielniczuk [email protected]
• Michael Aaron Murphy [email protected]

Closes #654 and #651

[rtzoeller]

The Haiku failure is an issue with the rust stdlib. I'll work on a fix.

https://github.com/rust-lang/rust/pull/97771