Add configurable bindir for distros without sbin

Adds a `BINDIR` Make variable, defaulting to `sbin`, allowing packagers
to override the install location of `beesd` for systems that do not use
`/sbin`.  This affects the install path and systemd unit template.

scripts/beesd@.service.in

@@ -5,7 +5,7 @@ After=sysinit.target
 
 [Service]
 Type=simple
-ExecStart=@PREFIX@/sbin/beesd --no-timestamps %i
+ExecStart=@PREFIX@/@BINDIR@/beesd --no-timestamps %i
 CPUAccounting=true
 CPUSchedulingPolicy=batch
 CPUWeight=12