Add shortcuts for "nexti" and "stepi" commands in Single-Key mode

Add shortcuts for "nexti" and "stepi" commands in Single-Key mode

Currently, "layout asm" is not so useful as "layout src" with Single-Key mode:
you have to use multi-key commands like "ni" and "si" to do single-stepping.
This patch adds, in addition to "next" and "step" commands, corresponding
assembly-level ones - "nexti" and "stepi" - to Single-Key mode, with the
shortcuts of "o" (from "step Over") and "i" (from "Step Into") respectively.

gdb/ChangeLog:

	* tui/tui.c (tui_commands): Add "nexti" and "stepi" to the Single-Key
	mode command list.

gdb/doc/ChangeLog:

	* gdb.texinfo (TUI Single Key Mode): Document the new shortcuts in
	Single-Key mode.