Add is_freenas function to check if we are FreeNAS or TrueNAS

(cherry picked from commit d5796dfb845754e6a5b57df6d905adbde8553b8b)
Use syslog dataset only for FreeNAS

Ticket:	#4391
(cherry picked from commit e51ed353ebb1e10c77fbe538727a3e76d0d1f1f6)