Remove the 'how' parameter to rsindex(), making it reverse-search for only the first matching character,
as sindex() already does. There are only two callers, and one already passed a hardcoded value of 1.
Also change rsindex() so that it starts searching at the character BEFORE the passed in position. This
makes it easier to repeatedly call rsindex() in a loop to search for the Nth matching character, and
also fixes a technical instance of undefined behaviour where a pointer is decremented to point before
the start of the string.
Remove the 'mark' parameter to strsearch(). Instead, always forward-search from the beginning of the
string and reverse-search from the end of the string, as this is what the two callers want anyway.
Bump the module ABI version because these functions are exported to modules.
ba1b9742ec