Revert "Process: do not inherit all file descriptors to child processes"

[shahmishal]

Reverts #2485

We are seeing failure on Ubuntu 18.04:

https://ci.swift.org/job/oss-swift-package-linux-ubuntu-18_04/2002/console

21:15:52 Test Case 'TestProcess.test_fileDescriptorsAreNotInherited' started at 2019-10-15 04:15:28.804
21:15:52 
/home/buildnode/jenkins/workspace/oss-swift-package-linux-ubuntu-18_04/swift-corelibs-foundation/TestFoundation/TestProcess.swift:587: error: TestProcess.test_fileDescriptorsAreNotInherited : XCTAssertTrue failed - 
21:15:52 /home/buildnode/jenkins/workspace/oss-swift-package-linux-ubuntu-18_04/swift-corelibs-foundation/TestFoundation/TestProcess.swift:588: error: TestProcess.test_fileDescriptorsAreNotInherited : XCTAssertEqual failed: ("5") is not equal to ("6") - 0
21:15:52 1
21:15:52 2
21:15:52 94
21:15:52 
21:15:52 Test Case 'TestProcess.test_fileDescriptorsAreNotInherited' failed (3.39 seconds)

cc: @weissi