MINOR: stconn: rename SE_FL_RXBLK_CONN to SE_FL_APPLET_NEED_CONN

MINOR: stconn: rename SE_FL_RXBLK_CONN to SE_FL_APPLET_NEED_CONN

This flag is exclusively used when a front applet needs to wait for the
other side to connect (or fail to). Let's give it a more explicit name
and remove the ambiguous function that was used only once.

This also ensures we will not risk to set it back on a new endpoint
after cs_reset_endp() via SE_FL_APP_MASK, because the flag being
specific to the endpoint only and not to the connector, we don't
want to preserve it when replacing the endpoint.