Make XRootDSource fork-safe

[nsmith-]

By setting the appropriate environment variable.

The bug this fixes is a hang in child processes triggered by use of uproot with XRootD files in multiprocessing or concurrent.futures process pools after doing so in the parent process. An example that triggers the bug is below:

#!/usr/bin/env python
import concurrent.futures
import uproot

file = "root://eospublic.cern.ch//eos/root-eos/cms_opendata_2012_nanoaod/Run2012B_DoubleMuParked.root"

def read(fn):
    print("starting work")
    with uproot.open(fn) as f:
        print("opened file")
        sumw = f['Events'].numentries
    return sumw

print(read(file))

with concurrent.futures.ProcessPoolExecutor(max_workers=1) as executor:
    future = executor.submit(read, file)
    print(future.result())

[nsmith-]

I couldn't find much documentation on why one would ever not want this option enabled. There is xrootd/xrootd#897

[jpivarski]

Unless you have any objection, I'll take this when the tests pass. It will combine with PR #269 to become uproot 0.5.1.