Cream grid jobs fail when no input or output files to transfer. #3415

Fix TransferRequest to behave intelligently when created with an empty
list of transfers. Before, it would issue a transfer command with "NULL"
for source and destination, which fails, causing Cream jobs to go on
hold.
Docs for cream file transfer bug. #3415
Added retries of transient glexec failures. #2415
Merge branch 'V7_8-branch'

Conflicts:
	src/condor_starter.V6.1/glexec_privsep_helper.linux.cpp
	src/condor_utils/param_info.in