jscan(8): Remove dupdata() (no longer used since 36d6bdee113b682f2).
libmagic: fix file identification

Removes local modifications from contrib code and
fixes at least the following identification issue.

Before:

    # file /usr/src/bin/test/test.c
    /usr/src/bin/test/test.c: ASCII text

After:

    # file /usr/src/bin/test/test.c
    /usr/src/bin/test/test.c: C source, ASCII text
file: libmagic knows about libz now so drop the dependency