gnuplot: Improve startup performance

Before executing the gnuplot executable the environment variable `GDFONTPATH`
is populated with a list of font directories, which is obtained from `fc-list`.
In that process we iterated over each line and called `dirname` on it, which
introduces a performance hit for loading and executing the external executable
`dirname` every time.

The new version avoids the loop.

The author of this patch measured a 42 fold performance improvement:

old version:

$ time ./gnuplot_old/bin/gnuplot -e ''
real 0m3.828s
user 0m0.392s
sys 0m0.465s

new version:

$ time ./gnuplot_new2/bin/gnuplot -e ''
real 0m0.091s
user 0m0.112s
sys 0m0.014s

The correctness of the value of `GDFONTPATH` was confirmed with the following
command and comparing its output between versions:

$ gnuplot -e 'print system("echo $GDFONTPATH")'

pkgs/tools/graphics/gnuplot/set-gdfontpath-from-fontconfig.sh

···
-p=( $(for n in $(fc-list | sed -r -e 's|^([^:]+):.*$|\1|'); do echo $(dirname "$n"); done | sort | uniq) )
+p=( $(fc-list : file | sed "s@/[^/]*: @@" | sort -u) )
 IFS=:
 export GDFONTPATH="${GDFONTPATH}${GDFONTPATH:+:}${p[*]}"
 unset IFS p