ls: use statx instead of stat when available

ls: use statx instead of stat when available

statx allows ls to indicate interest in only certain inode metadata.
This is potentially a win on networked/clustered/distributed
file systems. In cases where we'd have to do a full, heavyweight stat()
call we can now do a much lighter statx() call.

As a real-world example, consider a file system like CephFS where one
client is actively writing to a file and another client does an
ls --color in the same directory. --color means that we need to fetch
the mode of the file.

Doing that with a stat() call means that we have to fetch the size and
mtime in addition to the mode. The MDS in that situation will have to
revoke caps in order to ensure that it has up-to-date values to report,
which disrupts the writer.

This has a measurable affect on performance. I ran a fio sequential
write test on one cephfs client and had a second client do "ls --color"
in a tight loop on the directory that held the file:

Baseline -- no activity on the second client:

WRITE: bw=76.7MiB/s (80.4MB/s), 76.7MiB/s-76.7MiB/s (80.4MB/s-80.4MB/s),
       io=4600MiB (4824MB), run=60016-60016msec

Without this patch series, we see a noticable performance hit:

WRITE: bw=70.4MiB/s (73.9MB/s), 70.4MiB/s-70.4MiB/s (73.9MB/s-73.9MB/s),
       io=4228MiB (4433MB), run=60012-60012msec

With this patch series, we gain most of that ground back:

WRITE: bw=75.9MiB/s (79.6MB/s), 75.9MiB/s-75.9MiB/s (79.6MB/s-79.6MB/s),
       io=4555MiB (4776MB), run=60019-60019msec

* src/stat.c: move statx to stat struct conversion to new header...
* src/statx.h: ...here.
* src/ls.c: Add wrapper functions for stat/lstat/fstat calls,
and add variants for when we are only interested in specific info.
Add statx-enabled functions and set the request mask based on the
output format and what values are needed.
* NEWS: Mention the Improvement.