Static: variables in the "share" option.

This commit supports variable in the "share" option, the finding path to
file serve is the value from "share". An example:
{
    "share": "/www/data/static$uri"
}

docs/changes.xml

@@ -31,6 +31,19 @@ NGINX Unit updated to 1.26.0.
          date="" time=""
          packager="Andrei Belov <defan@nginx.com>">
 
+<change type="change">
+<para>
+the "share" option now specifies the entire path to the files it serves,
+rather than a document root directory to be prepended to the request URI.
+</para>
+</change>
+
+<change type="feature">
+<para>
+variables support in the "share" option.
+</para>
+</change>
+
 <change type="feature">
 <para>
 variables support in the "chroot" option.