Cleanup, fix warnings, move to 2018 edition

[josephlr]

This change does the following to get keyutils building without warnings on Rust 2018 edition.

• Adds edition = "2018" to both Cargo.tomls
• Cleans up Cargo.toml to:
  • Just use default versioning where they are equivalent to existing ~ and ^ patterns.
  • Have the documentation/homepage point the the correct thing.
• Fixup use statements to use crate or super where approprtiate.
• Use u32 and i32 instead of the libc types (as those are deprecated).
• Remove unnecessary use statements.

[mathstuf]

Thanks!