LU-16218 utils: add component flags "prefrd" and "prefwr"

LU-16218 utils: add component flags "prefrd" and "prefwr"

The initial implementation of "lfs setstripe ... --comp-flags=prefer"
only allowed specifying a single "prefer" argument for a given
mirror component, which would set both the "LCME_FL_PREF_RD" and
"LCME_FL_PREF_WR" flags at the same time.

This patch adds the separated component flags "prefrd" and "prefwr"
to allow setting the individual flags on a component.

Test-Parameters: trivial testlist=sanity-flr

Change-Id: I3e413cb37fab7ab2834946536705ce61a3feeed4
Signed-off-by: Jian Yu <yujian@whamcloud.com>