proftpd: use <pkg>_INSTALL_INIT_SYSV mechanism

Also use a complete destination path for the $(INSTALL) command.

[Peter: use install -D as noted by Yann]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>