serial: add routine to manipulate handshake state (RTS, DTR)

serial: add routine to manipulate handshake state (RTS, DTR)

Introduce a routine in libsigrok's internal serial layer which lets
applications manipulate the state of handshake signals (RTS and DTR)
after the serial port got opened and configured. This allows for timed
pulses which cannot get expressed with static "rts=1" etc phrases in
parameter strings, and allows handshake signal control while leaving
bitrate and frame format untouched. Applications specify which signals
to modify while other signals remain as they are (ternary input).

Do implement the signal manipulation in the libserialport transport,
do nothing and silently pass in the HID and BT transports. These can
get extended later as the need arises, depending on the HID chips' and
RFCOMM peers' capability to control these signals. This extension is
transparent to application code (acquisition device drivers).