implementation to replace single-character namespace prefixes, '$' is no
longer an invalid attribute name, and the namespace is relevant to
validity determination.
o Remove '$' case from ufs_extattr_valid_attrname()
o Add attrnamespace argument to ufs_extattr_valid_attrname(), and
fill out appropriately.
Currently no decisions are made based on the namespace argument, but
may be in the future.
Obtained from: TrustedBSD Project
25c2879cc9