USES=xfce: Stop overriding MASTER_SITE_SUBDIR.

Add an XFCE SITE_SUBDIR abbreviation and use it forf or the xfce ports,
which removes the need to set MASTER_SITE_SUBDIR in any of them.

This fixes ports that have USES=xfce but do not use the XFCE MASTER_SITE,
namely sysutils/xfce4-bsdcpufreq-plugin.

With hat: portmgr