libc: Add an implementation of strtol/strtoul and export it via plugins

These were lifted from the lua plugin.

sdl, doom, puzzles updated to use the exported version

todo: lua, maybe?
also: convert uses of atoi [back] to strtol

Change-Id: I5a1ebbe8d8c99349e594ab9bbbce474e7645b4e9

apps/plugin.c

@@ -571,6 +571,8 @@ static const struct plugin_api rockbox_api = {
     _ctype_,
 #endif
     atoi,
+    strtol,
+    strtoul,
     strchr,
     strcat,
     strlcat,