freebsd: include missing libc crate

[amshafer]

This causes compilation issues with the libc references in the devname_r definition in the devname function. With this compiling on freebsd succeeds.

Also make the non-linux versions of dev_path public, so other OS types can call it.

Here's the error:

error[E0433]: failed to resolve: use of undeclared crate or module `libc`
   --> src/node/mod.rs:215:9
    |
215 |         libc::devname_r(
    |         ^^^^ use of undeclared crate or module `libc`

error[E0433]: failed to resolve: use of undeclared crate or module `libc`
   --> src/node/mod.rs:217:13
    |
217 |             libc::S_IFCHR, // Must be S_IFCHR or S_IFBLK
    |             ^^^^ use of undeclared crate or module `libc`

error[E0433]: failed to resolve: use of undeclared crate or module `libc`
   --> src/node/mod.rs:229:31
    |
229 |     unsafe { dev_name.set_len(libc::strlen(buf)) };
    |                               ^^^^ use of undeclared crate or module `libc`

Fix is relatively simple although I'm not sure if you want me to limit the libc dependency in Cargo.toml to freebsd as well?

[Drakulix]

@amshafer Sorry for accidentally closing this, lets continue here #205