Treat environment variable values beginning with "T" as true

So a value is true iff it begins with 'T', 't', 'Y', or 'y'.  This is
mostly so that naively converting JSON "true" to a string will work
properly, but it should keep LISPers happy too.

misc.c

@@ -202,7 +202,7 @@ bool getenv_bool(const char *name, bool dflt)
 {
     char *env = getenv(name);
     if (env == NULL) return dflt;
-    if (env[0] == 'y' || env[0] == 'Y') return true;
+    if (strchr("yYtT", env[0])) return true;
     return false;
 }