Automatically run file command on remote connection to the target if NaCl
manifest was provided by nacl-manifest command before that.

All information about NaCl program is already in the manifest. So we shouldn't
require to enter this information twice.

We can't just execute file command whenever user issues nacl-manifest command
though since we don't know the program architecture until gdb will connect to
debug stub. That's why this should be done the moment we know the program
architecture (and file command doesn't fail) but before gdb tries to find
symbols in the main program. The only such moment when we can execute our code
is about_to_proceed event.

TEST= run chrome with --enable-nacl-debug --nacl-gdb=..., set breakpoints,
execute target remote command.

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