transfer: restore ability to define alternative/additional checksum

[DmitryLitvintsev]

Motivation:

Commit 3b12aa7 has introduced a side effect of theowing NPE when accessing getCellName() for JFR repoprting.

Modification:

Without going into much detail as to what is the root cause of NPE fuard Transfer JFR message source when cell address is unset

Result:

upload specifying alternative checksum works:

$ gfal-copy  -K md5:bed9c27139f6651fb9511da39210e9e8 --checksum-mode=target foo gsiftp://example.orgv/users/litvinse/foo.3
Copying file:///home/litvinse/foo   [DONE]  after 6s
[litvinse@uqbar ~]$ echo $?
0

and:

$ gfal-sum gsiftp://uqbar.fnal.gov/users/litvinse/foo.3 MD5
gsiftp://uqbar.fnal.gov/users/litvinse/foo.3 bed9c27139f6651fb9511da39210e9e8

$ gfal-sum gsiftp://uqbar.fnal.gov/users/litvinse/foo.3 ADLER32
gsiftp://uqbar.fnal.gov/users/litvinse/foo.3 be111976

Target: trunk
Request: 12.0, 11.2

Require-book: no
Require-notes: yes
Patch: https://rb.dcache.org/r/14724/
Acked-by: Marina
Require-book: no
Require-notes: yes