Added support for the read register (p) command.

The packet format of this command: p N, where N is the number of the register
number. The reply is the content of register N in target byte order.
Added support for the set breakpoint (Z0) command.

Packet format: Z0,addr,size, where addr is the address of the breakpoint and
size is its size.
Added support for the vCont? command.

The vCont? command is a request for what vCont commands are supported. We
advertise support for single stepping w/ and w/o signal and continue w/ and w/o
signal (none of these commands are implemented yet).
Added support for the vCont;c command and handling of breakpoints.

The vCont;c command instructs the target to continue execution. It is one
command in the sequence of commands needed to implement stepi. This commit also
includes a small test program that emulates the execution of a small loop.