fix Unix build with DEF_PAGER defined

Eliminate a couple of compile warnings produced when DEF_PAGER is
defined:  unixmain.c: g.catmore=DEF_PAGER; wintty.c: fd=open(...).

Override its use when DLB is also defined since an external pager
could access 'license' but not 'history', 'opthelp', &c when those
are in the dlb container file.

In the commented out value for DEF_PAGER, show a viable value for
the default configuration these days.

include/decl.h

@@ -838,7 +838,7 @@ struct instance_globals {
     int locknum; /* max num of simultaneous users */
 #endif
 #ifdef DEF_PAGER
-    char *catmore; /* default pager */
+    const char *catmore; /* external pager; from getenv() or DEF_PAGER */
 #endif
 #ifdef MICRO
     char levels[PATHLEN]; /* where levels are */

include/unixconf.h

@@ -93,10 +93,12 @@
 /*
  * Define DEF_PAGER as your default pager, e.g. "/bin/cat" or "/usr/ucb/more"
  * If defined, it can be overridden by the environment variable PAGER.
- * Hack will use its internal pager if DEF_PAGER is not defined.
- * (This might be preferable for security reasons.)
+ * NetHack will use its internal pager if DEF_PAGER is not defined _or_
+ * if DLB is defined since an external pager won't know how to access the
+ * contents of the dlb container file.
+ * (Note: leaving DEF_PAGER undefined is preferable for security reasons.)
  */
-/* #define DEF_PAGER ".../mydir/mypager" */
+/* #define DEF_PAGER "/usr/bin/less" */
 
 /*
  * Define PORT_HELP to be the name of the port-specfic help file.