Simplify calling of rsindex() and strsearch()

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.

Changelog

@@ -1,5 +1,7 @@
 [Changes 1.2.2]
 
+* Simplify rsindex() and strsearch() exported functions. (caf)
+
 * scr-bx now lists the detached screens if there is more than one that matches
   the string supplied by the user. (caf)