Fixed configure description for systemd-{unit,sysconfig}-dir

The usage information didn't match the actual configuration variables.

For example, the following message was given by ./configure
--with-systemd-unitdir:
configure: WARNING: unrecognized options: --with-systemd-unitdir

Another option is called "--with-init-dir", so I've decided to add a
dash in front of "dir".

refs #6710
Added install-systemd hint in Makefile

When calling make without parameters, the "install-systemd" option wasn't shown.

refs #6710

Signed-off-by: Michael Friedrich <michael.friedrich@netways.de>