conf/assemble: new config line "auto".

conf/assemble:  new config line "auto".

The line 'auto' in mdadm.conf can be used to disable assembly
of specific metadata types, or of all arrays.

This does not affect assembly of arrays listed in mdadm.conf
or on command line.

 auto -all

will disable all auto-assembly.

 auto -ddf

will cause mdadm to ignore ddf arrays that are not explicitly
mentioned, and auto assemble anything else it finds.

Signed-off-by: NeilBrown <neilb@suse.de>