core: add new setting Writable= to ListenSpecial= socket units

Writable= is a new boolean setting. If ture, then ListenSpecial= will
open the specified path in O_RDWR mode, rather than just O_RDONLY.

This is useful for implementing services like rfkill, where /dev/rfkill
is more useful when opened in write mode, if we want to not only save
but also restore its state.

man/systemd.socket.xml

@@ -381,6 +381,14 @@
         For SOCK_RAW the port is the IP protocol.</para></listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term><varname>Writable=</varname></term>
+        <listitem><para>Takes a boolean argument. May only be used in
+        conjunction with <varname>ListenSpecial=</varname>. If true,
+        the specified special file is opened in read-write mode, if
+        false in read-only mode. Defaults to false.</para></listitem>
+      </varlistentry>
+
       <varlistentry>
         <term><varname>MaxConnections=</varname></term>
         <listitem><para>The maximum number of connections to