Add a new function magic_load_buffers() which is analogous to magic_load().
Rather than a list of files it takes an array of buffers and an array of
their sizes.  Each buffer is assumed to be a compiled magic file.

This function is useful for strong compartmentalization where it is
useful to disallow direct filesystem access to the libmagic code.  Magic
data can instead be passed via shared memory or file handles.

Sponsored by:	DARPA, AFRL