systemd: use /etc/default/pcscd as EnvironmentFile

If you want to set environment variables to change the behavior of pcscd
you can use the file /etc/default/pcscd

For example you can add in this file:
PCSCLITE_FILTER_IGNORE_READER_NAMES="Twin"
so that readers with "Twin" in the name will be ignored.

See "Remove and/or customize PC/SC reader names"
https://ludovicrousseau.blogspot.com/2015/12/remove-andor-customize-pcsc-reader-names.html

Thanks to Maurizio Avogadro for the idea
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=989316

etc/pcscd.service.in

@@ -6,6 +6,7 @@ Documentation=man:pcscd(8)
 [Service]
 ExecStart=@sbindir_exp@/pcscd --foreground --auto-exit
 ExecReload=@sbindir_exp@/pcscd --hotplug
+EnvironmentFile=-@sysconfdir@/default/pcscd
 
 [Install]
 Also=pcscd.socket