Log Message:
kpathsea: update to 6.4.0

Part of TeXlive 2024

* Support an extended check for safe filenames which also allows writing under
  TEXMF[SYS]VAR, for Lua(LA)TeX; new functions and corresponding kpsewhich
  options.

* Support a new variable TEXMF_OUTPUT_DIRECTORY (alongside the traditional
  TEXMFOUTPUT), so that subprograms can have access to an --output-directory
  setting in an engine invocation.