Clean up the manpage and conditionalize various portions according to how

jemalloc is configured.

Modify arena_malloc() API to avoid unnecessary choose_arena() calls.  Remove
unnecessary code from choose_arena().

Enable lazy-lock by default, now that choose_arena() is both faster and out of
the critical path.

Implement objdir support in the build system.

jemalloc/README

@@ -0,0 +1,4 @@
+jemalloc is a general-purpose scalable concurrent malloc(3) implementation.
+
+The INSTALL file contains information on how to configure, build, and install
+jemalloc.