XrdCl: Make it possible for xrdcp to concatenate multiple sources to stdout

]==> xrdcp root://eospps.cern.ch//eos/ljanyst/testconcat/a048e67f-4397-4bb8-85eb-8d7e40d90763.dat root://eospps.cern.ch//eos/ljanyst/testconcat/b3d40b3f-1d15-4ad3-8cb5-a7516acb2bab.dat - | md5sum
Job: 1/2
Source: root://eospps.cern.ch:1094//eos/ljanyst/testconcat/a048e67f-4397-4bb8-85eb-8d7e40d90763.dat
Target: stdio://-
[1000MB/1000MB][100%][==================================================][90.91MB/s]
Job: 2/2
Source: root://eospps.cern.ch:1094//eos/ljanyst/testconcat/b3d40b3f-1d15-4ad3-8cb5-a7516acb2bab.dat
Target: stdio://-
[1000MB/1000MB][100%][==================================================][100MB/s]
2fcca11d452666b16fb46e29022dcccb  -

]==> cat a048e67f-4397-4bb8-85eb-8d7e40d90763.dat b3d40b3f-1d15-4ad3-8cb5-a7516acb2bab.dat | md5sum
2fcca11d452666b16fb46e29022dcccb  -