help_print_optstring: New table-based implementation.

help_print_optstring: New table-based implementation.

By moving the format strings into a single table and a table-based
lookup to select the matching string, we can drastically simplify
the control flow in this function.

This results in about a ~35% code size reduction compiled with gcc.

The tables are generated using gen-strtab.awk but we manually copy
it into the source file, as downstream users just pick out help.c
and help.h by themselves.