scsi: saner replacements for ->proc_info()

It's still an obsolete interface; don't introduce those in new drivers.
However, it's saner than the ->proc_info() and commits after this one
will convert the existing ->proc_info() users to it.

The read side is ->show_info(seq_file *, struct Scsi_Host *); use
seq_... for generating contents.

The write side is ->write_info(struct Scsi_Host *, char *, int).

Again, this is driven by procfs needs; we are going to kill ->write_proc()
and ->read_proc() and this is the main obstacle to burying that piece of
shit.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>