Fix bugs in libgateway and the AG, so the UG can read from it again:
* fill in the appropriate manifest fields in the AG disk-polling driver
* fill in the appropriate size fields
* return -1 on get_dataset (not 0, since this has a special value to
libmicrohttpd)
* clean up libgateway: remove POST and DELETE handlers (libgateway
is now effectively libAG, since the RG has its own handler
implementations).
* when marking an fs_entry stale, set its write_nonce to 0 so we force
the MS to generate new listing data.
* correctly generate the remote host:port in libsyndicate/httpd.cpp
Merge branch 'master' of https://github.com/jcnelson/syndicate

Conflicts:
	AG/drivers/disk_polling/driver.cpp