Factor out equivalent code from ttyread/ttysend

The echo-to-terminal portions of ttyread and ttysend were actually doing
the same thing.  New function twrite() now handles this.  The parameter
show_ctrl determines whether control characters are shown as "^A".  This
was the only difference between tputc and techo, and techo is now unused
and removed.

(This commit should not change st's behaviour.)

Signed-off-by: Devin J. Pohly <djpohly@gmail.com>

st.h

@@ -209,7 +209,7 @@ void selnormalize(void);
 int selected(int, int);
 char *getsel(void);
 
-size_t utf8decode(char *, Rune *, size_t);
+size_t utf8decode(const char *, Rune *, size_t);
 size_t utf8encode(Rune, char *);
 
 void *xmalloc(size_t);