Add ioctl(2) calls to uftdi(4) to access bitbang, MPSSE, CPU_FIFO, and
other modes supported by the FTDI serial adapter chips.

In addition to adding the new ioctls, this change removes all the code
that reset the chip at attach and open/close time, and also the code
that turned on RTS/CTS flow control on open without any permission to do
so (that was just always a bug in the driver).

When FTDI chips are configured as GPIO or MPSSE or other special-purpose
uses by an attached serial eeprom, the chip will power on with certain
pins driven or floating, and it's important that the driver not do
anything to the chip to perturb that unless it receives a specific
command to do so.  When used for "plain old serial comms" the chip
powers on into the right mode and never needs to be reset while it's
running to operate properly, so this change is transparent to most users.