Rename "set/show remotebaud" command into "set/show serial baud"

Rename "set/show remotebaud" command into "set/show serial baud"

This patch renames the "set/show remotebaud" commands into
"set/show serial baud", and moves its implementation into serial.c.
It also moves the "baud_rate" global from top.c to serial.c, where
the new code is being added (the alternative was to add an include
of target.h).

And to facilitate the transition to the new setting name, this
patch also preserves the old commands, and marks them as deprecated
to alert the users of the change.

gdb/ChangeLog:

        * cli/cli-cmds.c (show_baud_rate): Moved to serial.c as
        serial_baud_show_cmd.
        (_initialize_cli_cmds): Delete the code creating the
        "set/show remotebaud" commands.
        * serial.c (baud_rate): Move here from top.c.
        (serial_baud_show_cmd): Move here from cli/cli-cmds.c.
        (_initialize_serial): Create "set/show serial baud" commands.
        Add "set/show remotebaud" command aliases.
        * top.c (baud_rate): Moved to serial.c.
        * NEWS: Document the new "set/show serial baud" commands,
        replacing "set/show remotebaud".

gdb/doc/ChangeLog:

        * gdb.texinfo: Replace "set remotebaud" and "show remotebaud"
        by "set serial baud" and "show serial baud" (resp) throughout.