Add etc/X11/fontpath.d as a catalogue fontpath by default.

This allows a port/package to install a symlink pointing
the real font directory managed by core XLFD (aka fonts.dir).
Adding a new entry of "FontPath" into the system-wide
xorg.conf or "xset fp" by users manually is no longer needed.

When both USES=fonts and ${FONTPATHSPEC} are defined,
the post-install target will install ${FONTPATHSPEC} into
${FONTPATHD} directory as a symlink to ${FONTSDIR}.
The symlink name has the following syntax:

<identifier>:[attribute:]pri=<priority>

Details can be found in xorg.conf(5). A typical example
for a bitmap font is as follows:

FONTPATHSPEC=	${PORTNAME}:unscaled:pri=60

No objection on: x11