show: print list of available serial ports

show: print list of available serial ports

Introduce the --list-serial command line option, which prints the list
of serial ports detected by the sigrok serial.c abstraction.

The code was taken from Carl-Fredrik Sundström's <redacted>
message to the mailing list, with minor style fixes applied, and the
option renamed to explicitly mention its "list" nature, not suggesting
it would be a parameter to setup.