libc should provide libc::c_bool

[jimblandy]

The libc crate should export a type c_bool, defined to have the same representation as the C and C++ bool type on the current platform, analogous to c_int and so on.

[steveklabnik]

👍

[alexcrichton]

Closing along the lines of #128 which resulted in rust-lang/rfcs#954 (comment) (and discussion can be continued there)