Amendments for --follow mode in dmesg

Currently dmesg doesn't complain when --follow is given by non-root and CAP_SYSLOG is absent.
dmesg prints the log as usually but doesn't stay listening for new messages.
Proposed patch enables some self-diagnostics related to capabilities. --follow will now print nothing except errors if dmesg can't read /dev/kmsg (and can't actually follow because of that).
Build depends on cap-ng presence which is also required for setpriv. Please pay more attention for this point because my autotools experience is quite small.

I can also add ru/uk translations for the new dmesg prints. Should I contact language maintainers at translationproject.org/domain/util-linux for that?