Implemented goto_tab_nn using Alt+Fn like in Linux console

This may potentially break existing console applications which use Alt-Fn
for something.  In that case, the fix is to add the following to one's
.Xresources:

urxvt.tabbed.no-tabbedex-keys: yes
urxvt.keysym.Shift-Left: perl:tabbedex:prev_tab
urxvt.keysym.Shift-Right: perl:tabbedex:next_tab
urxvt.keysym.Shift-Down: perl:tabbedex:new_tab
urxvt.keysym.Shift-Up: perl:tabbedex:rename_tab
urxvt.keysym.Control-Left: perl:tabbedex:move_tab_left
urxvt.keysym.Control-Right: perl:tabbedex:move_tab_right

In case no-tabbedex-keys is already used, to enable goto_tab_nn bindings
the following needs to be added instead:

urxvt.keysym.Meta-F1: perl:tabbedex:goto_tab_1
urxvt.keysym.Meta-F2: perl:tabbedex:goto_tab_2
urxvt.keysym.Meta-F3: perl:tabbedex:goto_tab_3
urxvt.keysym.Meta-F4: perl:tabbedex:goto_tab_4
urxvt.keysym.Meta-F5: perl:tabbedex:goto_tab_5
urxvt.keysym.Meta-F6: perl:tabbedex:goto_tab_6
urxvt.keysym.Meta-F7: perl:tabbedex:goto_tab_7
urxvt.keysym.Meta-F8: perl:tabbedex:goto_tab_8
urxvt.keysym.Meta-F9: perl:tabbedex:goto_tab_9
urxvt.keysym.Meta-F10: perl:tabbedex:goto_tab_10
urxvt.keysym.Meta-F11: perl:tabbedex:goto_tab_11
urxvt.keysym.Meta-F12: perl:tabbedex:goto_tab_12

[mina86@mina86.com: Changed Meta-F<n> bindings to a simple loop
 and added more a detailed commit message.]
Change make_current to be a nop if changing to current tab.

This is important with goto_tab_nn command and thus the
Alt-F<n> bindings.
Make default tab names 1-based to match Fn keys