dmesg: fix and cleanup --read-clear

dmesg: fix and cleanup --read-clear

The function read_buffer() implements read and clear functionally, but
we do not differentiate between these actions in main() for error
messages, and one generic "dmesg: read kernel buffer failed" is used
in all cases. That's a bug.

This patch removes the "clear" action from read_buffer() and keeps it
for buffer reading only.  The "clear" action is implemented in main()
by separate klogctl(SYSLOG_ACTION_CLEAR) for cases. It means also for
"dmesg --read-clear"; we do not use SYSLOG_ACTION_READ_CLEAR anymore.

Now "clear+read" is:

 * syslog: SYSLOG_ACTION_READ_ALL + SYSLOG_ACTION_CLEAR
 * kmsg:   /dev/kmsg read()       + SYSLOG_ACTION_CLEAR

In old versions "dmesg --syslog --read-clear" (syalog backed) was
implemented by      logctl(SYSLOG_ACTION_READ_CLEAR) and it returns no
data for non-root  users (due to EPERM), "dmesg --read-clear" (kmsg)
returns data and EPERM for the "clear" action.

Now the command "dmesg --syslog --read-clear" and "dmesg --read-clear"
behaves in the same way -- returns data and EPERM for the "clear"
action.

Fixes: https://github.com/karelzak/util-linux/issues/1255
Signed-off-by: Karel Zak <kzak@redhat.com>