make BINDIR and SBINDIR availabe to input files

BINDIR and SBINDIR can be set independently of PREFIX.

Signed-off-by: Andrew Gregory <andrew.gregory.8@gmail.com>
replace @PREFIX@/sbin with @SBINDIR@

SBINDIR and BINDIR can be set independently of PREFIX.  This fixes
broken shebangs in service files when SBINDIR is set to something other
than PREFIX/sbin

Signed-off-by: Andrew Gregory <andrew.gregory.8@gmail.com>