New GDB/MI command "-info-gdb-mi-command"

This patch adds a new GDB/MI command meant for graphical frontends
trying to determine whether a given GDB/MI command exists or not.

Examples:

    -info-gdb-mi-command unsupported-command
    ^done,command={exists="false"}
    (gdb)
    -info-gdb-mi-command symbol-list-lines
    ^done,command={exists="true"}
    (gdb)

At the moment, this is the only piece of information that this
command returns.

Eventually, and if needed, we can extend it to provide
command-specific pieces of information, such as updates to
the command's syntax since inception.  This could become,
for instance:

    -info-gdb-mi-command symbol-list-lines
    ^done,command={exists="true",features=[]}
    (gdb)
    -info-gdb-mi-command catch-assert
    ^done,command={exists="true",features=["conditions"]}

In the first case, it would mean that no extra features,
while in the second, it announces that the -catch-assert
command in this version of the debugger supports a feature
called "condition" - exact semantics to be documented with
combined with the rest of the queried command's documentation.

But for now, we start small, and only worry about existance.
And to bootstrap the process, I have added an entry in the
output of the -list-features command as well ("info-gdb-mi-command"),
allowing the graphical frontends to go through the following process:

  1. Send -list-features, collect info from there as before;
  2. Check if the output contains "info-gdb-mi-command".
     If it does, then support for various commands can be
     queried though -info-gdb-mi-command. Newer commands
     will be expected to always be checked via this new
     -info-gdb-mi-command.

gdb/ChangeLog:

        * mi/mi-cmds.h (mi_cmd_info_gdb_mi_command): Declare.
        * mi/mi-cmd-info.c (mi_cmd_info_gdb_mi_command): New function.
        * mi/mi-cmds.c (mi_cmds): Add -info-gdb-mi-command command.
        * mi/mi-main.c (mi_cmd_list_features): Add "info-gdb-mi-command"
        field to output of "-list-features".

        * NEWS: Add entry for new -info-gdb-mi-command.

gdb/doc/ChangeLog:

        * gdb.texinfo (GDB/MI Miscellaneous Commands): Document
        the new -info-gdb-mi-command GDB/MI command.  Document
        the meaning of "-info-gdb-mi-command" in the output of
        -list-features.

gdb/testsuite/ChangeLog:

        * gdb.mi/mi-i-cmd.exp: New file.

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