[2/2] Fix font metric cache filename generation

[Wim Lewis]

This fixes a bug which has been driving me batty (probably the
same as bug #17674), in which Wine does not cache the font
metrics between runs. Turns out that MacOSX sets $DISPLAY
to /tmp/foo/:0.0, which causes Wine to try to cache metrics in a file
named ~/wine/cachedmetrics./tmp/foo/:0.0, which obviously doesn't work.

This patch just makes XFONT_UserMetricsCache() do a better job of
generating a unique but valid pathname from the value of $DISPLAY.

-- 
   Wim Lewis <wiml at hhhh.org>, Seattle, WA, USA. PGP keyID 27F772C1