textproc/xmlstarlet: Improve port and fix build with libxml2 2.14

• Switch from PORTVERSION to DISTVERSION as recommended by Porters Handbook
• Import patches from Debian for various fixes [1]
• Import patch from Gentoo to fix build with new versions of libxml2 and libxslt [2]
• Remove GNU_CONFIGURE_MANPREFIX
• Rework Makefile for better readability and to follow Porters Handbook more closely
• Unbreak with libxml2 2.14 [3]

References:

[1]
https://sources.debian.org/patches/xmlstarlet/1.6.1-4/50-fix-format-security.patch/
https://sources.debian.org/patches/xmlstarlet/1.6.1-4/60-avoid-silent-rules.patch/
https://sources.debian.org/patches/xmlstarlet/1.6.1-4/70-remove-link-usr-lib.patch/
https://sources.debian.org/patches/xmlstarlet/1.6.1-4/80-fix-unesc-dquot.patch/
https://sources.debian.org/patches/xmlstarlet/1.6.1-4/90-fix-upstream-version.patch/
https://sources.debian.org/patches/xmlstarlet/1.6.1-4/pkg-config.patch/

[2]
https://gitweb.gentoo.org/repo/gentoo.git/tree/app-text/xmlstarlet/files/xmlstarlet-1.6.1-clang17.patch

[3]
https://gitlab.archlinux.org/archlinux/packaging/packages/xmlstarlet/-/blob/34eb3ee24a6088a13d5de01af7a2f4e784cff0fc/PKGBUILD#L22

PR: 279976
Tested by: George Mitchell <george@m5p.com>
Approved by: maintainer timeout, 3 weeks