Introduce --enable-systemd (defaults to enabled)

Also replaces --with-systemddir with --with-systemdunitdir.
Both --with-systemdunitdir and --with-systemdpresetdir are ignored
if --disable-systemd is used.
Add --enable-sysvinit option (defaults to enabled)

Add --enable-sysvinit option to configure, allowing to skip installation
of the SysV init scripts by passing --disable-sysvinit. Defaults
to installing the SysV init scripts.
Honour results of --enable-systemd and --enable-sysvinit in etc/
Update zfs.spec file to new configure options

- add a systemd build conditional to enable systemd for distributions
  other than Fedora
- Update to newly supported configure options for systemd and SysV