sh: avoid warnings about too-long initializer strings

[dim]

Summary

Mark optletter and t_op::op_text as __non_string, to avoid
warnings from clang 21 similar to:

bin/sh/options.h:77:36: error: initializer-string for character array is too long, array size is 19 but initializer has size 20 (including the null terminating character); did you mean to use the 'nonstring' attribute? [-Werror,-Wunterminated-string-initialization]
   77 | const char optletter[NSHORTOPTS] = "efIimnsxvVECabupTPh";
      |                                    ^~~~~~~~~~~~~~~~~~~~~
bin/test/test.c:153:3: error: initializer-string for character array is too long, array size is 2 but initializer has size 3 (including the null terminating character); did you mean to use the 'nonstring' attribute? [-Werror,-Wunterminated-string-initialization]
  153 |         {"==",  STREQ},
      |          ^~~~

MFC after: 3 days