Simplify/improve idioms in usr.sbin/ntp Makefiles

• Use SRCTOP-relative paths to other directories instead of .CURDIR-relative ones. This simplifies pathing in make/displayed output.
• Also, use :H where possible/sensical to manipulate .CURDIR-relative paths
• Remove superfluous bsd.own.mk .includes which are already handled via src.opts.mk .includes

MFC after: 1 week
Sponsored by: Dell EMC Isilon