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.

INSTALL

@@ -128,15 +128,6 @@ any of the following arguments (not a definitive list) to 'configure':
     Statically link against the specified libunwind.a rather than dynamically
     linking with -lunwind.
 
---disable-munmap
-    Disable virtual memory deallocation via munmap(2); instead keep track of
-    the virtual memory for later use.  munmap() is disabled by default (i.e.
-    --disable-munmap is implied) on [64-bit] Linux, which has a quirk in its
-    virtual memory allocation algorithm that causes semi-permanent VM map holes
-    under normal jemalloc operation.  Although munmap() causes issues on 32-bit
-    Linux as well, it is not disabled by default due to the practical
-    possibility of address space exhaustion.
-
 --disable-fill
     Disable support for junk/zero filling of memory.  See the "opt.junk" and
     "opt.zero" option documentation for usage details.