always use "%s"-style format for printf()-style functions

[trofi]

ncuses-6.3 added printf-style function attributes and now makes it easier to catch cases when user input is used in palce of format string when built with CFLAGS=-Werror=format-security:

src/lib-std.c:3623:32: error: format not a string literal and no format arguments [-Werror=format-security]
 3623 |             ERROR0(ERR_WARNING,errmsg);
      |                                ^~~~~~

Let's wrap all the missing places with "%s" format.