Use ttyname_r on BSDs

[Artoria2e5]

ttyname_r is a POSIX function for getting the name of any TTY, including a PTY, in an reentrant way. A POSIX (and nix::libc) compliant declaration is found in OpenBSD (bitrig too?), DragonFly, NetBSD, and FreeBSD -- so basically every value of target_os except Windows and the can be covered now.

Pseudocode:

1. Check via isatty that fd is a tty. If not, error. (ttyname_r will complain later too.)
2. Use sysconf to get TTY_NAME_MAX as sz. If not present or error, assume a default of 128.
3. Allocate a buffer of the size sz.
4. Call ttyname_r(fd, buf, sz).
5. Tame the C string.

A quick of this thing is that some implementations return the old style /dev/ttyp name.

Don't use it on the mac -- gnulib says Apple fucked the declaration up.

[nelsonjchen]

Hmm, it's not available in Nix. Maybe this should be added there. Would they be game?

https://docs.rs/nix/0.17.0/nix/?search=ttyname

[Artoria2e5]

The libc crate it re-exports has ttyname_r: https://docs.rs/libc/0.2.66/libc/fn.ttyname_r.html (with the usual unsafe and errno territory warnings.) It uses a POSIX-compliant link name on macs too.

And yeah, it would be a good idea to wrap it safely...

[Artoria2e5]

nix-rust/nix#1204