can't pass more than 1 control bit for TCP protocol

[getxsick]

TCP(control_bits='fin')

but we can't pass more than one control bit to constructor.

we can do like this:

x = TCP()
x.set_flags('control_bits', fin=True, psh=True)

Propably in Protocol.set_field() should be extra checking if the field isn't a flag type and then call Protocol.set_flag()

[getxsick]

fixed r4924