Implement array_search for all values (return array of keys)

[remicollet]

I notice there is no simple way to search all values in an array (to retrieve all the keys)

Here is a simple implementation.

I also add a ZVAL_STR_OR_LONG which can perhaps be moved in Zend, as probably used in lot of places.

7.1 seems a good candidate for a very minor self-contained change (else 7.2/master is fine)

[cmb69]

Perhaps a new function array_search_all() would be better (I find boolean arguments hard to understand, and we're already having $strict).

[remicollet]

We already have array_keys(array, search) (thanks Nikita)