MINOR: conn_stream: add new sets of functions to set/get endpoint flags

MINOR: conn_stream: add new sets of functions to set/get endpoint flags

At plenty of places we need to manipulate the conn_stream's endpoint just
to set or clear a flag. This patch adds a handful of functions to perform
the common operations (clr/set/get etc) on these flags at both the endpoint
and at the conn_stream level.

The functions were named after the target names, i.e. se_fl_*() to act on
the stream endpoint flags, and sc_ep_* to manipulate the endpoint flags
from the stream connector (currently conn_stream).

For now they're not used.