Update how custom baudrates are handled on OS X

This is taken from the example serial program
on Apple's developer website, see:

http://free-pascal-general.1045716.n5.nabble.com/Non-standard-baud-rates-in-OS-X-IOSSIOSPEED-IOCTL-td4699923.html
Merge pull request #33 from wjwwood/fix_custom_baudrate_osx

Update how custom baudrates are handled on OS X