nohup: avoid FORTIFY runtime failure on Bionic libc

nohup: avoid FORTIFY runtime failure on Bionic libc

The meaning of non-file permission umask bits is implementation defined.
On Bionic libc, attempting to set them triggers a FORTIFY runtime check.

  $ nohup true
  FORTIFY: umask: called with invalid mask -601
  Aborted                    nohup true

* src/nohup.c: (main) Avoid setting non-permission bits in umask.
Just clear the umask to ensure we create nohup.out with u+rw,
as we restore the original umask before the exec().
* tests/misc/nohup.sh: Add a test case.
* NEWS: Mention the bug fix.