handle bz (bzip not bzip2) better

in fact .tbz is tar.bz (bzip1) and .tbz2 is what tar.bz2 is used
commonly.

i modified the code, but did not run any tests (not automated unittests or uploads with the ui), haven't figured out yet how to run them

anyway, seems the two bytes of both compressor match signature used in dokuwiki and php-bz2 extension opens the stream without errors:

    $ tar -cf /tmp/a.tar.bz --use-compress=bzip .gitignore 
    $ tar -cf /tmp/a.tar.bz2 --use-compress=bzip2 .gitignore 

    $ file /tmp/a.tar.bz*
    /tmp/a.tar.bz:  data
    /tmp/a.tar.bz2: bzip2 compressed data, block size = 900k

    $ cut -c1-4 /tmp/a.tar.bz | od -cx
    0000000   B   Z   0   9  \n   `  \0  \n
               5a42    3930    600a    0a00
    0000010
    $ cut -c1-4 /tmp/a.tar.bz2 | od -cx
    0000000   B   Z   h   9  \n  \t   % 032 017  \n   J  \f 311 206  \n   X
               5a42    3968    090a    1a25    0a0f    0c4a    86c9    580a
    0000020   >   j   *  \n 367   w   m 031  \n
               6a3e    0a2a    77f7    196d    000a
    0000031

    $ php -r '$f=bzopen("/tmp/a.tar.bz", "r"); var_dump($f);'|less
    resource(4) of type (stream)
    $ php -r '$f=bzopen("/tmp/a.tar.bz2", "r"); var_dump($f);'|less
    resource(4) of type (stream)