Fix legacy SLES provisioning paths

[viniciusferrao]

Summary

This follow-up restores legacy SLES provisioning paths after the openSUSE Leap support work landed upstream.

• Keep SLES 11 UEFI xNBA on the existing elilo path instead of direct EFI-stub boot.
• Select elilo for SLES 11 UEFI AutoYaST stateful installs.
• Precreate SLES 11 mypostscript for install and netboot paths.
• Fix SLES netboot image generation for older init behavior and preserve genimage failures.
• Add missing SLES 12 xfsprogs package coverage.
• Harden postscript retrieval failures and fix legacy openssl detection.
• Handle BIND 9.18 named configuration syntax.
• Add focused unit/static coverage for SLES 11 UEFI boot handling.

Validation

Local checks:

• git diff --check origin/master..HEAD
• Perl syntax checks for changed xCAT plugin files
• Shell syntax checks for applicable changed scripts
• PERL5LIB=xCAT-server/lib/perl:perl-xCAT perl xCAT-test/unit/xnba_semicolon.t
• PERL5LIB=xCAT-server/lib/perl:perl-xCAT perl xCAT-test/unit/sles_pre_script.t
• Sensitive-string scan on changed files

perlcritic still reports the same legacy plugin findings as origin/master; the normalized finding set is unchanged.

Lab/KVM validation:

• SLES 11 old head node, BIOS stateless: pass
• SLES 11 old head node, UEFI stateless: pass
• SLES 11 old head node, BIOS stateful: pass
• SLES 11 old head node, UEFI stateful: pass
• SLES 12 UEFI stateless regression smoke: pass
• SLE 15/openSUSE Leap UEFI stateless regression smoke: pass

Generated assets/configuration were regenerated before live boot validation where applicable (genimage, packimage, nodeset, makedhcp).