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.

gdb/ChangeLog

@@ -1,3 +1,8 @@
+2017-08-16  Ruslan Kabatsayev  <b7.10110111@gmail.com>
+
+	* tui/tui.c (tui_commands): Add "nexti" and "stepi" to the Single-Key
+	mode command list.
+
 2017-08-15  Stafford Horne  <shorne@gmail.com>
 
 	* MAINTAINERS (Write After Approval): Add Stafford Horne.

gdb/doc/ChangeLog

@@ -1,3 +1,8 @@
+2017-08-16  Ruslan Kabatsayev  <b7.10110111@gmail.com>
+
+	* gdb.texinfo (TUI Single Key Mode): Document the new shortcuts in
+	Single-Key mode.
+
 2017-08-12  Sergio Durigan Junior  <sergiodj@redhat.com>
 
 	PR gdb/21925

gdb/doc/gdb.texinfo

@@ -25415,6 +25415,10 @@ finish
 @item n
 next
 
+@kindex o @r{(SingleKey TUI key)}
+@item o
+nexti.  The shortcut letter @samp{o} stands for ``step Over''.
+
 @kindex q @r{(SingleKey TUI key)}
 @item q
 exit the SingleKey mode.
@@ -25427,6 +25431,10 @@ run
 @item s
 step
 
+@kindex i @r{(SingleKey TUI key)}
+@item i
+stepi.  The shortcut letter @samp{i} stands for ``step Into''.
+
 @kindex u @r{(SingleKey TUI key)}
 @item u
 up

gdb/tui/tui.c

@@ -75,8 +75,10 @@ static const struct tui_char_command tui_commands[] = {
   { 'd', "down" },
   { 'f', "finish" },
   { 'n', "next" },
+  { 'o', "nexti" },
   { 'r', "run" },
   { 's', "step" },
+  { 'i', "stepi" },
   { 'u', "up" },
   { 'v', "info locals" },
   { 'w', "where" },