mg: have Insert key toggle overwrite mode by default

[Lari Rasku]

There's a fairly strong convention among text editors that the Insert
key should toggle overwrite mode.  This is admittedly far more common
among GUI editors, but could mg adopt it as a default anyway?

diff --git usr.bin/mg/ttykbd.c usr.bin/mg/ttykbd.c
index 67bc8e4bd..485291a77 100644
--- usr.bin/mg/ttykbd.c
+++ usr.bin/mg/ttykbd.c
@@ -52,6 +52,8 @@ ttykeymapinit(void)
                dobindkey(fundamental_map, "scroll-up", key_npage);
        if (key_ppage)
                dobindkey(fundamental_map, "scroll-down", key_ppage);
+       if (key_ic)
+               dobindkey(fundamental_map, "overwrite-mode", key_ic);
        if (key_dc)
                dobindkey(fundamental_map, "delete-char", key_dc);