[5.1/mi] Enable MI interface

[Andrew Cagney]

Hello,

A 5.1 release criteria is enabling the MI interface.

Once my other MI questions get resolved, the biggest problem I can see
with this is with the MI testsuite.  Once MI is enabled the testsuite
will kick in and, most likely, some targets will have additional
failures.

If anyone can see any problems with this, please start rasing them now
:-)  The change required to enable the MI is trivial.  The consequences
may not be :-)

For reference, the testsuite output on a BSD UNIX system looks like:

Running /home/scratch/GDB/src/gdb/testsuite/gdb.mi/mi-basics.exp ...
Running /home/scratch/GDB/src/gdb/testsuite/gdb.mi/mi-break.exp ...
Running /home/scratch/GDB/src/gdb/testsuite/gdb.mi/mi-console.exp ...
FAIL: gdb.mi/mi-console.exp: Hello message (timeout)
Running /home/scratch/GDB/src/gdb/testsuite/gdb.mi/mi-disassemble.exp
...
Running /home/scratch/GDB/src/gdb/testsuite/gdb.mi/mi-eval.exp ...
Running /home/scratch/GDB/src/gdb/testsuite/gdb.mi/mi-hack-cli.exp ...
Running /home/scratch/GDB/src/gdb/testsuite/gdb.mi/mi-read-memory.exp
...
Running /home/scratch/GDB/src/gdb/testsuite/gdb.mi/mi-regs.exp ...
Running /home/scratch/GDB/src/gdb/testsuite/gdb.mi/mi-return.exp ...
Running /home/scratch/GDB/src/gdb/testsuite/gdb.mi/mi-simplerun.exp ...
Running /home/scratch/GDB/src/gdb/testsuite/gdb.mi/mi-stack.exp ...
FAIL: gdb.mi/mi-stack.exp: stack args listing 0
FAIL: gdb.mi/mi-stack.exp: stack args listing 1
FAIL: gdb.mi/mi-stack.exp: stack info-depth
FAIL: gdb.mi/mi-stack.exp: stack info-depth 99
Running /home/scratch/GDB/src/gdb/testsuite/gdb.mi/mi-stepi.exp ...
Running /home/scratch/GDB/src/gdb/testsuite/gdb.mi/mi-until.exp ...
Running /home/scratch/GDB/src/gdb/testsuite/gdb.mi/mi-var-block.exp ...
Running /home/scratch/GDB/src/gdb/testsuite/gdb.mi/mi-var-child.exp ...
Running /home/scratch/GDB/src/gdb/testsuite/gdb.mi/mi-var-cmd.exp ...
Running /home/scratch/GDB/src/gdb/testsuite/gdb.mi/mi-var-display.exp
...
Running /home/scratch/GDB/src/gdb/testsuite/gdb.mi/mi-watch.exp ...
FAIL: gdb.mi/mi-watch.exp: wp out of scope (2)

                === gdb Summary ===

# of expected passes            486
# of unexpected failures        6
# of expected failures          14

Hmm, thinking about it, 500 additional tests is pretty impressive!  Well
done Elena and Fernando!

	Andrew