Add terminfo entries for backspace mode

St used to use backspace as BS until the commit 230d0c8, but due
to general lack of knowledge of lusers, we moved to the most common
configuration in linux to avoid answering the same question 3 times
per month. With the most common configuration we have a backspace
that returns a DEL, and we have a Delete key that doesn't return a
DEL character neither a BS.

When dealing with devices connected using a serial line (or even
with Plan9) it is more common Backspace as BS and Delete as DEL. For
this reason, st is not always the best tool when you talk with a
serial device.

This patch adds new terminfo entries for Backspace as BS and Delete
as DEL. A patch for confg.h is also added, to make easier switch
between both configurations.

st.info

@@ -220,3 +220,13 @@ st-meta-256color| simpleterm with meta key and 256 colors,
 	smm=\E[?1034h,
 	rs2=\E[4l\E>\E[?1034h,
 	is2=\E[4l\E>\E[?1034h,
+
+st-bs| simpleterm with backspace as backspace,
+	use=st,
+	kbs=\010,
+	kdch1=\177,
+
+st-bs-256color| simpleterm with backspace as backspace and 256colors,
+	use=st-256color,
+	kbs=\010,
+	kdch1=\177,