couple of DECgraphics optimizations

If a termcap entry for ending attributes and color also contains
the code to switch from secondary font back to primary (HE_resets_AS
hack), maybe strip the AS code out of HE (and clear the HE_resets_AS
flag) when setting up DECgraphics.  Affects whether nethack sends
extra AS sequences while rendering a run of VT line-drawing chars.
My HE doesn't reset AS so that aspect hasn't been exercized.

When switching back and forth between normal and line-drawing,
defer the switch away from line-drawing if the character will be
rendered the same in both character sets (uppercase letter, digit,
most punctuation).  That might just defer the AE, but could skip it
and next AS depending on what characters are written.  The cycle
might repeat an arbitrary number of times, avoiding sending many
AS+AE combinations rather than just one.

Both of these optimizations are pretty small but reducing the number
of characters sent from a server to a remote user is worthwhile.

win/tty/termcap.c

@@ -399,6 +399,13 @@ tty_decgraphics_termcap_fixup(void)
 #if defined(ASCIIGRAPH) && !defined(NO_TERMS)
     /* some termcaps suffer from the bizarre notion that resetting
        video attributes should also reset the chosen character set */
+    if (dynamic_HIHE) {
+        (void) strsubst(nh_HE, AE, "");
+        (void) strsubst(nh_HE, ctrlO, "");
+    }
+    /* if AE has prefixing, substituting empty string for it probably
+       didn't work; however, if that _did_ work, we'll be able to
+       avoid HE_resets_AS and the degraded performance it causes */
     {
         const char *nh_he = nh_HE, *ae = AE;
         int he_limit, ae_length;
@@ -426,6 +433,16 @@ tty_decgraphics_termcap_fixup(void)
             ++nh_he, --he_limit;
         }
     }
+    /* some termcaps have AS load the line-drawing character set as
+       primary instead of having initialization load it as secondary
+       (we've already done that init) and then having AS simply switch
+       to secondary (change to do that now); they also have AE load
+       the US character set, which we avoid by not touching primary
+       [this speedup won't happen if they have delay prefixing though] */
+    if (!strcmp(AS, "\033(0") && !strcmp(AE, "\033(B")) {
+        AS = ctrlN;
+        AE = ctrlO;
+    }
 #endif
 }
 #endif /* TERMLIB */