Include GDB/MI docs in GDB manual

[Dmitry Sivachenko]

I think it is not good thing to include gdbmi.texinfo into 
GDB manual.  It contains very technical issues, which regular users
would not be happy to see in the Manual.
GDB manual should contain things which describe using of the debugger
from the users's point of view.

Probably it should be better included into gdbint.texinfo.

Or left as a separate book.

--dima