tests: fix false failure in nohup.sh in non tty builds

* tests/misc/nohup.sh: When running tests without a controlling tty,
an exec failure is triggered in a subshell, which causes POSIX
shells to immediately exit the subshell.  This was brought
to notice by the newly conforming bash 4.3.
Fixes http:/bugs.gnu.org/16940
tests: avoid the :> construct which can hide errors

On most shells `:>file || framework_failure_` will not evaluate
the framework_failure_ even if there was an error writing the file.
shells which do evaluate the failure are ksh 93u+ and bash 4.2,
while shells wich don't include bash 4.3, solaris, freebsd, dash.

Furthermore this construct is problematic on Solaris 10 sh,
which will try to optimize away a `:' command in a loop
after the first iteration, even if it is redirected.

* tests/cp/link-deref.sh: Remove the leading colon on redirections.
* tests/cp/reflink-perm.sh: Likewise.
* tests/id/zero.sh: Likewise.
* tests/install/install-C.sh: Likewise.
* tests/misc/env.sh: Likewise.
* tests/misc/md5sum-bsd.sh: Likewise.
* tests/misc/runcon-no-reorder.sh: Likewise.
* tests/mv/partition-perm.sh: Likewise.
* tests/rm/r-root.sh: Likewise.
* tests/split/l-chunk.sh: Likewise.
* tests/split/line-bytes.sh: Likewise.
* tests/tail-2/inotify-rotate.sh: Likewise.
* tests/tail-2/retry.sh: Likewise.
* tests/tail-2/symlink.sh: Likewise.
* tests/tail-2/wait.sh: Likewise.
* tests/touch/read-only.sh: Likewise.
+ cfg.mk (sc_prohibit_colon_redirection): A new syntax check
to avoid further instances of this creeping in.