possible memory leak with sessions

[LeSpocky]

The global variable sess_fname is a pointer to dynamically allocated memory which happens either in sess_generate_id() or in cgi_session_start(). However this memory is never freed.

[LeSpocky]

This is not easy to solve. sess_fname is currently a static variable in session.c and the only API function exposed in the headers which could do this is cgi_end() which is implemented in cgi.c.

Exposing the variable globally: bad. Exposing the variable to cgi.c only: not possible without exposing it globally (?).

This whole concept of global variables in a library is broken, but for v1.x we can't change it anymore, if we want to maintain API compatibility.

I propose adding a new function like cgi_session_free() which does the cleanup.

[LeSpocky]

Moved this to milestone v1.2, because introducing a new function would extend the API which would trigger a minor version bump with semantic versioning.