style: tweak tab after #define advice

Once upon a time, #define<tab> was cultural thing. However, even when it
was promulgated, it was a minority usage. 20 years ago the split was
30k/69k (tab/space) and today the split is 80k/546k (tab/space). Update
guidance to allow either with the usual suggestion to be consistent
within a file.

Reviewed by: sef, allenjude, 0mp (prior rev), jhb
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D30254