Close open transactions soon to avoid wraparound problems

Vacuumdb is giving me these messages:

Vacuuming public.files...
WARNING: oldest xmin is far in the past
HINT: Close open transactions soon to avoid wraparound problems.
WARNING: oldest xmin is far in the past
HINT: Close open transactions soon to avoid wraparound problems.
VACUUM

Two questions:
1. Is there a way for me to see what application/host/user has open
transactions that need to be closed?
2. If #1 is not possible then what can I do about this?

Searching the web via normal methods only turns up indexed source code
files for vacuum.

What's been done so far:
-Cluster-wide vacuum (these are done nightly, I ran one again just for gp)
-Ran the following query looking for old/suspicious
connections/transactions, didn't find anything useful:

select procpid, datname, client_addr, usename, current_query,
query_start from pg_stat_activity order by query_start asc;

Thanks in advance,
Brian