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.

include/jemalloc/internal/jemalloc_preamble.h.in

@@ -98,13 +98,6 @@ static const bool maps_coalesce =
     false
 #endif
     ;
-static const bool config_munmap =
-#ifdef JEMALLOC_MUNMAP
-    true
-#else
-    false
-#endif
-    ;
 static const bool config_stats =
 #ifdef JEMALLOC_STATS
     true