[mszeredi@suse.cz] rewrite and split big patch into managable chunks

/proc/mounts in its current form lacks important information:

 - propagation state
 - root of mount for bind mounts
 - the st_dev value used within the filesystem
 - identifier for each mount and it's parent

It also suffers from the following problems:

 - not easily extendable
 - ambiguity of mountpoints within a chrooted environment
 - doesn't distinguish between filesystem dependent and independent options
 - doesn't distinguish between per mount and per super block options

This patch introduces /proc/<pid>/mountinfo which attempts to address
all these deficiencies.

Code shared between /proc/<pid>/mounts and /proc/<pid>/mountinfo is
extracted into separate functions.

Thanks to Al Viro for the help in getting the design right.
Signed-off-by: Ram Pai <linuxram@us.ibm.com>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>