Don't imply that all action values can be OR'd.

This is neither POSIX compliant nor what the implementation does.
This could be allowed by changing the value of TCSAFLUSH from 2 to 3,
but that doesn't seem worthwhile after 25+ years.

Reviewed by: imp
MFC after: 1 week
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D25659