MINOR: session: expose session_embryonic_build_legacy_err() function

MINOR: session: expose session_embryonic_build_legacy_err() function

rename session_build_err_string() to session_embryonic_build_legacy_err()
and add new <out> buffer argument to the prototype. <out> will be used as
destination for the generated string instead of implicitly relying on the
trash buffer. Finally, expose the new function through the header file so
that it becomes usable from any source file.

The function is expected to be called with a session originating from
a connection and should not be used for applets.