cli: strip path separators from extracted binary tag filenames

[aizu-m]

Extracting a binary tag item to a file (cover art, wvunpack -xx "Field=%t") names the output after the item's embedded filename. That name is untrusted: it is the tag value bytes up to the first NUL, straight from the .wv/APEv2 tag. dump_tag_item_to_file() runs it through filespec_name() first, but filespec_name() only removes the platform's native separator:

embedded name:  ../../../../../ab.jpg
filespec_name(), _WIN32 build  ->  ../../../../../ab.jpg
filespec_name(), POSIX build   ->  ab.jpg

Windows treats both / and \ as separators, so on Windows a forward-slash name passes through unchanged and reaches fopen() as the output name, writing the file above the target directory. POSIX has the mirror gap: an embedded \ survives (harmless there, but it is the same routine). I found it by crafting a cover-art tag whose embedded name held separators and watching where the extracted file landed.

Reduce the embedded name to its final component, checking both separators, before it becomes the output name. Normal filenames are unaffected. wvtag keeps an identical copy of the function and is fixed the same way.