GDBServer: give more complete usage information

GDBServer: give more complete usage information

--attach/--multi are currently only mentioned on the usage info first
lines, the meaning of PROG is completely absent and the COMM text does
not mention '-/stdio'.

A few options are missing:

 . --disable-randomization / --no-disable-randomization is not mentioned.

Although the manual has a comment saying these are superceded by
QDisableRandomization, that only makes sense for "run" in
extended-remote mode.  When we start gdbserver passing it a PROG,
--disable-randomization / --no-disable-randomization do take effect.
So I think we should document these.

 . We show --debug / --remote-debug, so might as well show --disable-packet too.

GDB's --help has this "For more information, consult the GDB manual"
blurb that is missing in GDBserver's --help.

Then shuffle things around a bit into "Operating modes", "Other
options" and "Debug options" sections, similarly to GDB's --help
structure.

Before:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
$ ./gdbserver/gdbserver --help
Usage:  gdbserver [OPTIONS] COMM PROG [ARGS ...]
        gdbserver [OPTIONS] --attach COMM PID
        gdbserver [OPTIONS] --multi COMM

COMM may either be a tty device (for serial debugging), or
HOST:PORT to listen for a TCP connection.

Options:
  --debug               Enable general debugging output.
  --debug-format=opt1[,opt2,...]
                        Specify extra content in debugging output.
                          Options:
                            all
                            none
                            timestamp
  --remote-debug        Enable remote protocol debugging output.
  --version             Display version information and exit.
  --wrapper WRAPPER --  Run WRAPPER to start new programs.
  --once                Exit after the first connection has closed.
Report bugs to "<http://www.gnu.org/software/gdb/bugs/>".
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

After:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
$ ./gdbserver/gdbserver --help
Usage:  gdbserver [OPTIONS] COMM PROG [ARGS ...]
        gdbserver [OPTIONS] --attach COMM PID
        gdbserver [OPTIONS] --multi COMM

COMM may either be a tty device (for serial debugging),
HOST:PORT to listen for a TCP connection, or '-' or 'stdio' to use
stdin/stdout of gdbserver.
PROG is the executable program.  ARGS are arguments passed to inferior.
PID is the process ID to attach to, when --attach is specified.

Operating modes:

  --attach              Attach to running process PID.
  --multi               Start server without a specific program, and
                        only quit when explicitly commanded.
  --once                Exit after the first connection has closed.
  --help                Print this message and then exit.
  --version             Display version information and exit.

Other options:

  --wrapper WRAPPER --  Run WRAPPER to start new programs.
  --disable-randomization
                        Run PROG with address space randomization disabled.
  --no-disable-randomization
                        Don't disable address space randomization when
                        starting PROG.

Debug options:

  --debug               Enable general debugging output.
  --debug-format=opt1[,opt2,...]
                        Specify extra content in debugging output.
                          Options:
                            all
                            none
                            timestamp
  --remote-debug        Enable remote protocol debugging output.
  --disable-packet=opt1[,opt2,...]
                        Disable support for RSP packets or features.
                          Options:
                            vCont, Tthread, qC, qfThreadInfo and
                            threads (disable all threading packets).

For more information, consult the GDB manual (available as on-line
info or a printed manual).
Report bugs to "<http://www.gnu.org/software/gdb/bugs/>".
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

gdb/gdbserver/ChangeLog:
2015-04-01  Pedro Alves  <palves@redhat.com>
	    Cleber Rosa  <crosa@redhat.com>

	* server.c (gdbserver_usage): Reorganize and extend the usage
	message.