sfeed_update: use xargs -P -0

Some of the options, like -P are as of writing (2023) non-POSIX:
https://pubs.opengroup.org/onlinepubs/9699919799/utilities/xargs.html. However
many systems support this useful extension for many years now.

Some historic context:

The xargs -0 option was added on 1996-06-11, about a year after the NetBSD
import (over 27 years ago at the time of writing):

http://cvsweb.openbsd.org/cgi-bin/cvsweb/src/usr.bin/xargs/xargs.c?rev=1.2&content-type=text/x-cvsweb-markup

On OpenBSD the xargs -P option was added on 2003-12-06 by syncing the FreeBSD
code:

http://cvsweb.openbsd.org/cgi-bin/cvsweb/src/usr.bin/xargs/xargs.c?rev=1.14&content-type=text/x-cvsweb-markup

Looking at the imported git history log of GNU findutils (which has xargs), the
very first commit already had the -0 and -P option on Sun Feb 4 20:35:16 1996
+0000.

Tested on many systems, old and new, some notable:

- OpenBSD 7.4
- Void Linux
- FreeBSD 12
- NetBSD 9.3
- HaikuOS (uses GNU tools).
- Slackware 11
- OpenBSD 3.8
- NetBSD 5.1

Some shells:
- oksh
- bash
- dash
- zsh

During testing there are some incompatibilities found in parsing the fields so
the arguments are passed as one argument which is split later on by the child
program.

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