Added support for Mac OS

[gonium]

Hi,

the pull request contains an implementation of the serial code for mac os. Basically I just took the linux code and changed some minor details:

• Some Baudrates are not defined on Mac OS, so I removed them They're baudrates above 115200 baud which I did not encounter in real life so far.
• The definitions for TCGETS and TCSETS differ.
• Minor adjustments such as replacing uint32 w/ uint64

I also changed the test code to use different PTYs. This is probably not what is reasonable - the testcode depends on the runtime system (since it depends on PTY names). Probably you could refactor the test to use environment variables, but this decision is up to you.

The testsuite works, as well as my small test program for getting values from a SDM630 smart meter. I use socat just like under linux for running the testsuite.

-Mathias

[nqv]

Thank you @gonium We should later merge mac and linux code into shared unix one.