databases/xtrabackup8: update to version 8.0.31-24

This change includes files/patch-llvm-15, a copy of the patch
fixing UB in MySQL Server 8.0.31 code included in xtrabackup8
and discovered with Clang 15:

https://cgit.freebsd.org/ports/tree/databases/mysql80-server/files/patch-llvm15-fix?id=9f33a321916b

The version 8.0.31-24 includes auto-tuning of its "use-memory" option
that needs implementation of host_free_memory() but lacks one
suitable for FreeBSD. This change adds simple implementation by me
in files/patch-storage_innobase_xtrabackup_src_utils.cc
that sums sysctls vm.stats.vm.v_free_count and vm.stats.vm.v_inactive_count
assuming that FreeBSD VM subsystem frees Inactive pages as needed.

This feature was tested by port maintainer Eugene Zheganin
successfully performing backup of 730GB MySQL InnoDB
that failed with previous xtrabackup8 version if used with defaults.

Drop unused dependency on vim and explicitly disable
online version update check. Force NDEBUG build as it fails otherwise.
Pet portlint.

Work-around a problem with ncurses-6.3 dependency that installs
wrong libncurses.so file (ASCII text), so use libncurses.so.6
in LIB_DEPENDS instead.

Tested by: Eugene Zheganin
Approved by: Eugene Zheganin (maintainer)