Add MG_EV_SSI_CALL_CTX

This is the variant of MG_EV_SSI_CALL with context of the SSI call being
processed in addition to the tag argument. Specifically, call handler
now gets access to the HTTP request and the name of the file being processed.

MG_EV_SSI_CALL is preserved for backward compatibility.

PUBLISHED_FROM=eeea7487c062505abdb23b6f64994ff7a6b60ff8