[FEATURE] Unset parts in t3lib_div::array_merge_recursive_overrule()

Add possibility to t3lib_div::array_merge_recursive_overrule()
to unset values from the resulting array using special values
__UNSET in the second array.

Change-Id: Iab17d59b80c1e1070c8b2db6b318da0b01fda13c
Resolves: #33941
Releases: 4.7
Reviewed-on: http://review.typo3.org/9026
Reviewed-by: Oliver Klee
Reviewed-by: Tolleiv Nietsch
Tested-by: Tolleiv Nietsch
Reviewed-by: Susanne Moog
Tested-by: Susanne Moog