Fix with custom SYSCONF_DIR and fix qa tests

Change the home directory of the quagga user to /var/empty which prevent QA
First the home does not need to be harcoded to /usr/local/etc/quagga, second
it fixes issues where the home created by the user addition is 755 then the
plist modifies it to 750

Reviewed by: mat
Approved by: mat (maintainer)
Sponsored by: gandi.net
Differential Revision: https://reviews.freebsd.org/D3135