plugins: use stricter parsing for option values

also: convert the stored int value from 'int' to 's64'

atoi fails silently, returning a zero. instead we use the more robust
strtoll which will allow us fail with an error.

we also make the parsing for bools stricter, only allowing plausibly
boolean values to parse.

lightningd/plugin.h

@@ -114,7 +114,7 @@ struct plugins {
  */
 struct plugin_opt_value {
 	char *as_str;
-	int *as_int;
+	s64 *as_int;
 	bool *as_bool;
 };