paccache: Support passing extra arguments to the paccache service from a "config" file

Robin Candaurequested to merge
antiz/pacman-contrib:extra_arg_paccache_service into master

Add the generic pacman-contrib "config" file under /etc/conf.d (so it can be used for other purposes than paccache in the future), allowing to pass extra arguments to the paccache service via the "PACCACHE_ARGS" environment variable.

Closes #133 (closed)

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