UNPACK_DIR should only strip .tar as 2nd suffix

Fixes the problem where UNPACK_DIR returned rtmpdump-2 instead
of rtmpdumo-2.3 for rtmpdump-2.3. Only .tar.* archives have two
filetype suffixes, tgz, zip,.. have only one and the second "."
in the filename belongs to the version number which should not be
stripped.
contrib: Add librtmp build
contrib: Enable rtmp support in ffmpeg/libav