Move terminal echo logic into st.c

The only thing differentiating ttywrite and ttysend was the potential
for echo; make this a parameter and remove ttysend.

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

st.h

@@ -176,8 +176,7 @@ void tsetdirtattr(int);
 void ttynew(char *, char *, char **);
 size_t ttyread(void);
 void ttyresize(int, int);
-void ttysend(char *, size_t);
-void ttywrite(const char *, size_t);
+void ttywrite(const char *, size_t, int);
 
 void resettitle(void);