fix unicode glitch in DCS strings, patch by Tim Allen

Reported on the mailinglist:

"
I discovered recently that if an application running inside st tries to
send a DCS string, subsequent Unicode characters get messed up. For
example, consider the following test-case:

    printf '\303\277\033P\033\\\303\277'

...where:

  - \303\277 is the UTF-8 encoding of U+00FF LATIN SMALL LETTER Y WITH
    DIAERESIS (ÿ).
  - \033P is ESC P, the token that begins a DCS string.
  - \033\\ is ESC \, a token that ends a DCS string.
  - \303\277 is the same ÿ character again.

If I run the above command in a VTE-based terminal, or xterm, or
QTerminal, or pterm (PuTTY), I get the output:

    ÿÿ

...which is to say, the empty DCS string is ignored. However, if I run
that command inside st (as of commit 9ba7ecf), I get:

    ÿÿ

...where those last two characters are \303\277 interpreted as ISO8859-1
characters, instead of UTF-8.

I spent some time tracing through the state machines in st.c, and so far
as I can tell, this is how it works currently:

  - ESC P sets the "ESC_DCS" and "ESC_STR" flags, indicating that
    incoming bytes should be collected into the strescseq buffer, rather
    than being interpreted.
  - ESC \ sets the "ESC_STR_END" flag (when ESC is received), and then
    calls strhandle() (when \ is received) to interpret the collected
    bytes.
  - If the collected bytes begin with 'P' (i.e. if this was a DCS
    string) strhandle() sets the "ESC_DCS" flag again, confusing the
    state machine.

If my understanding is correct, fixing the problem should be as easy as
removing the line that sets ESC_DCS from strhandle():

diff --git a/st.c b/st.c
index ef8abd5..b5b805a 100644
--- a/st.c
+++ b/st.c
@@ -1897,7 +1897,6 @@ strhandle(void)
		xsettitle(strescseq.args[0]);
		return;
	case 'P': /* DCS -- Device Control String */
-		term.mode |= ESC_DCS;
	case '_': /* APC -- Application Program Command */
	case '^': /* PM -- Privacy Message */
		return;

I've tried the above patch and it fixes my problem, but I don't know if
it introduces any others.
"

st.c

@@ -2060,7 +2060,6 @@ strhandle(void)
 		xsettitle(strescseq.args[0]);
 		return;
 	case 'P': /* DCS -- Device Control String */
-		term.mode |= ESC_DCS;
 	case '_': /* APC -- Application Program Command */
 	case '^': /* PM -- Privacy Message */
 		return;