Add FreeBSD support

[valpackett]

Should also add OpenBSD and NetBSD, but I don't have them running on anything right now…

[dcuddeback]

Nice! Thanks for doing this.

[dcuddeback]

The way the crate is organized, the constants exported here are supposed to be fairly cross platform. It was previously exporting just the POSIX-defined constants, but I didn't realize that BSD doesn't define some of them. I doubt any of the other constants are in common use, so I think it would be best to remove the other constants here for all OS's so that the interface remains the same on all platforms. The others will still be accessible through termios::os. At least then it's clear for a user when they're using a constant that won't compile on all platforms. This will be a breaking change, but I can bump the version to v0.2.

[valpackett]

Makes sense!

Here's man termios from different operating systems:

FreeBSD:

           OPOST   /* enable following output processing */
           ONLCR   /* map NL to CR-NL (ala CRMOD) */
           OCRNL   /* map CR to NL */
           TABDLY  /* tab delay mask */
           TAB0    /* no tab delay and expansion */
           TAB3    /* expand tabs to spaces */
           ONOEOT  /* discard EOT's ‘^D’ on output) */
           ONOCR   /* do not transmit CRs on column 0 */
           ONLRET  /* on the terminal NL performs the CR function */

OpenBSD:

           OPOST    /* enable following output processing */
           ONLCR    /* map NL to CR-NL (ala CRMOD) */
           OXTABS   /* expand tabs to spaces */
           ONOEOT   /* discard EOT's (^D) on output */
           OCRNL    /* map CR to NL */
           OLCUC    /* translate lower case to upper case */
           ONOCR    /* No CR output at column 0 */
           ONLRET   /* NL performs the CR function */ 

NetBSD:

          OPOST      /* enable following output processing */
          ONLCR      /* map NL to CR-NL (ala CRMOD) */
          OCRNL      /* map CR to NL */
          OXTABS     /* expand tabs to spaces */
          ONOEOT     /* discard EOT's (^D) on output */
          ONOCR      /* do not transmit CRs on column 0 */
          ONLRET     /* on the terminal NL performs the CR function */ 

OS X:

           OPOST     /* enable following output processing */
           ONLCR     /* map NL to CR-NL (ala CRMOD) */
           OXTABS    /* expand tabs to spaces */
           ONOEOT    /* discard EOT's `^D' on output) */
           OCRNL     /* map CR to NL */
           OLCUC     /* translate lower case to upper case */
           ONOCR     /* No CR output at column 0 */
           ONLRET    /* NL performs CR function */

The least common subset seems to be OPOST, ONLCR, OCRNL, ONOEOT, ONOCR, ONLRET.

And OXTABS which is named TAB3 on FreeBSD for some reason :D

[dcuddeback]

Looks great. Thanks!