api

Sending or receiving trickle messages required setting flags in BOINC_OPTIONS.
There were two problems with this:
1) it wasn't documented
2) it's not necessary; the act of calling boinc_send_trickle_up()
   tells the runtime system to do the trickle-up-related stuff.

Furthermore, because intermediate file upload shares message channels
with trickles, these functions also required the option flags
(also undocumented).

With this change, you don't need to set options to use
trickle messages are intermediate file upload.