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.

doc/jemalloc.xml.in

@@ -650,6 +650,16 @@ for (i = 0; i < nbins; i++) {
         during build configuration.</para></listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term>
+          <mallctl>config.mremap</mallctl>
+          (<type>bool</type>)
+          <literal>r-</literal>
+        </term>
+        <listitem><para><option>--enable-mremap</option> was specified during
+        build configuration.</para></listitem>
+      </varlistentry>
+
       <varlistentry>
         <term>
           <mallctl>config.munmap</mallctl>