chmem: do not word a configuration failure as an instruction to the user

chmem: do not word a configuration failure as an instruction to the user

When the current machine does not support configuring/deconfiguring
memory, simply report that it cannot be done, instead of seemingly
telling the user to skip the configuring/deconfiguring.

Also, there is no need to mention `chmem` in the message: mentioning
just the recommended option (-e or -d) is enough.  Also, consistently
terminate all four messages with a period.

This improves upon commit ab2e709600 ("chmem: improve messages").

CC: Sumanth Korikkar <sumanthk@linux.ibm.com>
Signed-off-by: Benno Schulenberg <bensberg@telfort.nl>