lsblk: inform about depencency to /sys/dev/block

lsblk: inform about depencency to /sys/dev/block

The lsblk depends on /sys/dev/block/ symlinks, which appeared in
kernel 2.6.27.  Users with old, or non-sysfs configured, kernel
got ealier message

lsblk: md0: failed to initialize sysfs handler
lsblk: xvda: failed to initialize sysfs handler
[...]

that I found a little too difficult to understand.  This patch will
change the message to

lsblk: failed to access sysfs directory: /sys/dev/block: No such file or directory

and informs in manual page what could be reason to that.

[kzak@redhat.com: - use access() rather than opendir()]

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
Signed-off-by: Karel Zak <kzak@redhat.com>