serial_libsp: move libserialport specific code to separate source file

Introduce a new serial_libsp.c source file, and move code from serial.c
there which is specific to libserialport. Keep the existing serial.c API
in place, this is a pure internal refactoring.

Adjust a little whitespace while we are here. Rearrange long lines to
keep related parameter groups adjacent (like pointer and size, or UART
frame length and flow control). Consistently reduce indentation of
continuation lines.