Replace --disable-munmap with opt.munmap.

Control use of munmap(2) via a run-time option rather than a
compile-time option (with the same per platform default).  The old
behavior of --disable-munmap can be achieved with
--with-malloc-conf=munmap:false.

This partially resolves #580.

src/extent.c

@@ -1123,7 +1123,7 @@ extent_alloc_retained(tsdn_t *tsdn, arena_t *arena,
 			extent_gdump_add(tsdn, extent);
 		}
 	}
-	if (!config_munmap && extent == NULL) {
+	if (!opt_munmap && extent == NULL) {
 		extent = extent_grow_retained(tsdn, arena, r_extent_hooks,
 		    new_addr, size, pad, alignment, slab, szind, zero, commit);
 	}