Use mremap(2) for huge realloc().

If mremap(2) is available and supports MREMAP_FIXED, use it for huge
realloc().

Initialize rtree later during bootstrapping, so that --enable-debug
--enable-dss works.

Fix a minor swap_avail stats bug.

.gitignore

@@ -18,4 +18,6 @@
 /jemalloc/test/*.[od]
 /jemalloc/test/*.out
 /jemalloc/test/[a-z]*
+!/jemalloc/test/*.c
+!/jemalloc/test/*.exp
 /jemalloc/VERSION