Fix 'install: symlink usr/src/sys -> /sys: File exists' in distributeworld

[lwhsu]

Summary

Follow r334617, also specify ${DISTDIR} & '/base' while installing '/sys' symbolic link.

Test Plan

cd src && make -DNO_ROOT distributeworld

[lwhsu]

The full correct path also needs '/base'

[lwhsu]

Add ${INSTALLFLAGS} to provide required arguments for install(1)
Remove slash between ${DESTDIR}/${DISTDIR} to prevent superfluous slash in path

[markj]

Approved, but I suggest waiting a day for reviewers to respond. I don't understand this area well enough to review it myself.

[brd]

The only concern I have here is the use of DISTDIR which is also used in ports to override the location of Distfiles..

[lwhsu]

In D16877#359737, @brd wrote:

The only concern I have here is the use of DISTDIR which is also used in ports to override the location of Distfiles..

I think this is fine, DISTDIR is used in lots of places in this file.

BTW, after looking this file again, it seems that using INSTALL_DDIR which is defined earlier in this file is the proper way to have sensible ${DESTDIR}/${DISTDIR}. I'll work on a patch to utilize that.

[lwhsu]

Use ${INSTALL_DDIR} to prevent consecutive slash caused by
${DESTDIR}/${DISTDIR}