MFV r275783:

Convert ARC flags to use enum. Previously, public flags are defined in
arc.h and private flags are defined in arc.c which can lead to confusion
and programming errors.

Consistently use 'hdr' (when referencing arc_buf_hdr_t) instead of 'buf'
or 'ab' because arc_buf_t are often named 'buf' as well.

Illumos issue:

5369 arc flags should be an enum
5370 consistent arc_buf_hdr_t naming scheme

MFC after: 2 weeks