Fix Margay_TimeSet to be fully OSX compatible

[bschulz1701]

Issue:

Code does not trigger into OSX condition (Len > 17) because it always trips at Ubuntu condition Len > 11, also substring usage is incorrect withing these functions. Needs to be substring(0, x) not simply, substring(x)

Solution:

Clarify OSX conditional, change sub-strings, test for use of cu vs tty port for OSX

• Fix conditional
• Fix substrings
• Clarify cu vs. tty
• Test on OSX
• Test on Ubuntu