from the gobug.st/serial package and subject to the conditions of its
license (which has been reproduced in source as per terms of said
license)

Updated serial_posix to support full list of termios baud rates (as per
existing serial_linux implementation).

Moved posixTimeoutValues method from top level serial.go to new
serial_x.go source file, as only required for linux and posix
implementations.

Added ListPorts method in serial.go that delegates to platform
implementation called listPorts. Associated godoc explicitly states that posix
implementation is just a convenient heuristic. Renamed platform-specific
implementations in serial_posix.go and serial_windows.go from
GetPortsList to listPorts for consistency.

Reverted baud-rate additions in serial_posix.go. May be included a
future PR.

Corrected erroneous exporting of function in serial_windows.go

Tidied up posix implementation of listPorts.

Externalised portname filter definition to posix_<platform>.go