misc: if dlopen() is available, use it to load the libmagic library

misc: if dlopen() is available, use it to load the libmagic library

This prevents the libmagic library from being a hard dependency; if
it's not there, we can simply do without it.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: Darrick J. Wong <darrick.wong@oracle.com>