LU-15743 utils: add --xattr option to lfs find

LU-15743 utils: add --xattr option to lfs find

This adds a new "[!] --xattr" option to lfs find to enable listing
files that match a given extended attribute. The option takes an
argument in the form "NAME[=VALUE]" where NAME is a regular
expression for the attribute name and VALUE is an optional regular
expression to match the named attribute's value. If the option is
negated, only files that do not match the option are listed.

The provided regular expressions must match the entire name or value,
not just a substring. If only NAME is provided, files will match if
they have an extended attribute matching the name, regardless of the
attribute's contents. The option may be specified multiple times, and
files must match every provided argument in this case.

Signed-off-by: Thomas Bertschinger <bertschinger@lanl.gov>
Change-Id: I7b02e704b741ee30387a827dd5a25a20574cc3df
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/52804
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Alexandre Ioffe <aioffe@ddn.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>