Log Message:
Update tex-xkeyval to 2.6a.

Changes since 2.5f:
- General: Altered policy for handling \XKV@rm in nested \setkeys*
  commands: all unknown keys will be recorded, not only the once from
  the outermost \setkeys*
- \XKV@s@tk@ys: Removed \global again for consistent approach of
  \XKV@rm and to allow groups to keep \XKV@rm local
- \XKV@setkeys: Avoid reset of \XKV@rm in nested \setkeys commands
- \XKV@srstate: Removed XKV@rm again on user request
- \@s@lective@sanitize: protecting assignments from #
- General: Added \KV@def to keyval.tex as it is used by some packages
- \XKV@addtomacro@n: protecting assignments from #
- \XKV@addtomacro@o: protecting assignments from #
- \XKV@ch@cksanitize: protecting assignments from #
- \XKV@f@r: protecting assignments from #
- \XKV@for@n: protecting assignments from #
- \XKVV@t@bulate: Solved bug occurring with empty prefix
- \@s@lective@sanitize: Use private scratch register
- \XKV@addtomacro@n: Use private scratch register
- \XKV@addtomacro@o: Use private scratch register
- \XKV@ch@cksanitize: Use private scratch register
- \XKV@f@r: Use private scratch register
- \XKV@for@n: Use private scratch register