community/fcron: improve

• Set fifodir to /run to be align with piddir (the default is $localstatedir/run aka /var/run)
• Simplify init script, allow customization of command_args (I'm using the name $CRON_OPTS here as it is also used in crond and cronie)
• Correct init script permission to 0755 (was 0775)
• Use /var/spool/fcron as fcron user HOME (to be align with cron user from alpine-baselayout package)
• Recompile systab file on post-install and post-upgrade events (in the occasion where the systab file is never edited, the crontabs in there will never be run). Recompiling the file will create a /var/spool/fcron/systab file registered to fcron daemon.