Add some copyright headers where missing.
Add more MythSystem unit tests.
Eliminate GetSignal() from new MythSystem.

The ability to query the signal that killed a program is not part of POSIX,
so we can't even depend on it on all unix systems. GetExitCode() will now
return -1 if a program was killed by a signal, -2 if it is still uncollected
and 0..255 if the program exited normally, so GetSignal() is unnecessary as
a call if all it tells us is whether a signal killed the program.
Eliminate dead MythSystem flag kMSBuffered.

IO is always buffered.
Add more MythSystem unit tests.
Add annotations to three flags to eliminate if possible.
Eliminate unused kMSInUi MythSystem mask
Eliminate unused kMSSetPGID MythSystem mask