Curses: fix extended command input

The extended command input prompt was behaving in an unintended way:
Typing #a<enter> executed #adjust. Spaces in the entry prevented matching
any command. No error message was given when no command was matched.

Fix all of those, so it behaves more like the tty.

Clean up the tty, curses, and X11 windowport code, so they don't use
the extcmdlist array directly, but query with extcmds_match
and extcmds_getentry.

src/cmd.c

@@ -2449,6 +2449,58 @@ static const struct {
 
 int extcmdlist_length = SIZE(extcmdlist) - 1;
 
+/* get entry i in the extended commands list. for windowport use. */
+struct ext_func_tab *
+extcmds_getentry(int i)
+{
+    if (i < 0 || i > extcmdlist_length)
+        return 0;
+    return &extcmdlist[i];
+}
+
+/* find extended command entries matching findstr.
+   if findstr is NULL, returns all available entries.
+   returns: number of matching extended commands,
+            and the entry indexes in matchlist.
+   for windowport use. */
+int
+extcmds_match(const char *findstr, int ecmflags, int **matchlist)
+{
+    static int retmatchlist[SIZE(extcmdlist)] = DUMMY;
+    int i, mi = 0;
+    int fslen = findstr ? strlen(findstr) : 0;
+    boolean ignoreac = (ecmflags & ECM_IGNOREAC) != 0;
+    boolean exactmatch = (ecmflags & ECM_EXACTMATCH) != 0;
+    boolean no1charcmd = (ecmflags & ECM_NO1CHARCMD) != 0;
+
+    for (i = 0; extcmdlist[i].ef_txt; i++) {
+        if (extcmdlist[i].flags & (CMD_NOT_AVAILABLE|INTERNALCMD))
+            continue;
+        if (!wizard && (extcmdlist[i].flags & WIZMODECMD))
+            continue;
+        if (!ignoreac && !(extcmdlist[i].flags & AUTOCOMPLETE))
+            continue;
+        if (no1charcmd && (strlen(extcmdlist[i].ef_txt) == 1))
+            continue;
+        if (!findstr) {
+            retmatchlist[mi++] = i;
+        } else if (exactmatch) {
+            if (!strcmpi(findstr, extcmdlist[i].ef_txt)) {
+                retmatchlist[mi++] = i;
+            }
+        } else {
+            if (!strncmpi(findstr, extcmdlist[i].ef_txt, fslen)) {
+                retmatchlist[mi++] = i;
+            }
+        }
+    }
+
+    if (matchlist)
+        *matchlist = retmatchlist;
+
+    return mi;
+}
+
 const char *
 key2extcmddesc(uchar key)
 {