Do not install NIS related headers when MK_NIS == "no"

[garga]

Summary

include/rpcsvc headers are always being installed. Isolate NIS related
headers and don't install them when MK_NIS is defined.

Add these files to OptionalObsoleteFiles.inc so they are removed by
delete-old

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=196197

[ngie]

The change by itself is good, but please be sure to run this through an exp- run and make sure to run “make buildworld” on a system without these headers and MK_NIS==no.

[garga]

In D19701#422022, @ngie wrote:

The change by itself is good, but please be sure to run this through an exp- run and make sure to run “make buildworld” on a system without these headers and MK_NIS==no.

buildworld on a system without those headers didn't work.

/usr/src/lib/libc/rpc/auth_des.c:54:10: fatal error: 'rpcsvc/nis.h' file not found
#include <rpcsvc/nis.h>
         ^~~~~~~~~~~~~~
1 error generated.

I'll need some time to take a look at it. The way it is today is breaking ports like samba when WITHOUT_NIS=yes is set. samba considers NIS is installed because it finds headers