MINOR: cli: define usermsgs print context

MINOR: cli: define usermsgs print context

CLI 'add server' handler relies on usermsgs_ctx to display errors in
internal function on CLI output. This may be also extended to other
handlers.

However, to not clutter stderr from another contextes, usermsgs_ctx must
be resetted when it is not needed anymore. This operation cannot be
conducted in the CLI parse handler as display is conducted after it.

To achieve this, define new CLI states CLI_ST_PRINT_UMSG /
CLI_ST_PRINT_UMSGERR. Their principles is nearly identical to states for
dynamic messages printing.