stmhal: Fix HAL error raising; make test for it.

Addresses issue #968.
stmhal: Improve CAN init so that it can take sjw, bs1, bs2 args.

Also update docs to explain how CAN baudrate is determined.
docs: Fix RTD build configuration.

ReadTheDocs needs the root directory in its search path so it finds
topindex.html.
stream: Convert .ioctl() to take fixed number of args.

This is more efficient, as allows to use register calling convention.
If needed, a structure pointer can be passed as argument to pass more
data.
stream: Implement seek operation support via ioctl, wrapped in generic method.

Also, implement for unix port.
tests: Add test for file.seek().
stmhal: Switch to file.seek() implementation using stream ioctl.
ports: Define mp_off_t.