There were some unneeded codeblocks and TODO/XXX comments in the code
that have been removed. All issues marked with TODO/XXX have either been
already resolved or tracking issues exist.
This commit expands the conditional compilation directives to support
building Alacritty for OpenBSD. The build succeeds, and Alacritty runs
without issue once https://github.com/rust-lang/libc/pull/957 has been
merged and added to a versioned libc release.
This has been tested on the recently-released OpenBSD 6.3 on amd64 with
rustc 1.24.0 from its standard ports tree.
Because there are so many clippy warnings in the current codebase,
this commit removes '#![cfg_attr(feature = "clippy", deny(clippy))]',
to make it easier to fix warnings incrementally.
