MINOR: tools: add errname to print errno macro name

MINOR: tools: add errname to print errno macro name

Add helper to print the name of errno's corresponding macro, for example
"EINVAL" for errno=22. This may be helpful for debugging and for using in
some CLI commands output. The switch-case in errname() contains only the
errnos currently used in the code. So, it needs to be extended, if one starts
to use new syscalls.