Parses a comma-separated list of options of the form key or key=value or key="value".
Commas and equals-signs are only permitted in the key="value" case. In the case of
key=value1,key=value2 collisions, the last value wins. Returns a hashmap of key/value pairs,
or EINVAL in the case of malformed input. Note that no escape character sequence is supported,
so values may not contain the " character.