Add the --enable-mremap option.

Add the --enable-mremap option, and disable the use of mremap(2) by
default, for the same reason that freeing chunks via munmap(2) is
disabled by default on Linux: semi-permanent VM map fragmentation.

INSTALL

@@ -108,6 +108,12 @@ any of the following arguments (not a definitive list) to 'configure':
     released in bulk, thus reducing the total number of mutex operations.  See
     the "opt.tcache" option for usage details.
 
+--enable-mremap
+    Enable huge realloc() via mremap(2).  mremap() is disabled by default
+    because the flavor used is specific to Linux, which has a quirk in its
+    virtual memory allocation algorithm that causes semi-permanent VM map holes
+    under normal jemalloc operation.
+
 --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.