Support disabling sorting function and template parameters.

This commit adds a new XSL parameter boost.sort.params, which allows
users to disable alphabetical sorting of the function and template
parameters in detailed descriptions of functions and classes. By
default, sorting is enabled, which maintains the previous behavior.

Sorting is disabled when the parameter is set to 0. In this case,
parameters are listed in the order they are listed in the BoostBook
documentation, which is normally the same as they are listed in the
function signature or template preamble.

xsl/function.xsl

@@ -24,6 +24,10 @@
        unless everything fits on a single line. -->
   <xsl:param name="boost.short.result.type">12</xsl:param>
 
+  <!-- When true, the stylesheet will sort parameters of functions and
+       templates alphabetically in detailed description. -->
+  <xsl:param name="boost.sort.params">1</xsl:param>
+
   <!-- Display a function declaration -->
   <xsl:template name="function">
     <xsl:param name="indentation"/>
@@ -822,7 +826,7 @@
                   list-presentation="table"
                 </xsl:processing-instruction>
                 <xsl:for-each select="parameter|signature/parameter">
-		              <xsl:sort select="attribute::name"/>
+                  <xsl:sort select="attribute::name[$boost.sort.params!=0]"/>
                   <xsl:if test="description">
                     <varlistentry>
                       <term>
@@ -853,7 +857,7 @@
                 </xsl:processing-instruction>
                 <xsl:for-each select="template/template-type-parameter|
                       template/template-nontype-parameter">
-                  <xsl:sort select="attribute::name"/>
+                  <xsl:sort select="attribute::name[$boost.sort.params!=0]"/>
                   <xsl:if test="purpose">
                     <varlistentry>
                       <term>