Revert "phlsys_fs: remove unused *_file_lock_context()"

Turns out that atomic_replace_text_file() doesn't work well on NFS at
all, go back to locks on local filesystem.

This reverts commit 230922ee8e2255b7eef611b7532bd3c598df82bb.
Revert "Arcyd: write status files w/ atomic swap not lock"

Turns out that atomic_replace_text_file() doesn't work well on NFS at
all, go back to locks on local filesystem.

This reverts commit 5636816ce8b767f093e2174459ca3040ad62659a.
Revert "phlsys_fs: Add atomic_replace_text_file()"

Turns out that atomic_replace_text_file() doesn't work well on NFS at
all, go back to locks on local filesystem.

Keep the tidy to use for-loop in 'testbed/reader-writer/run.sh'.

This reverts commit 43662d2c0cca9d8b567b2387eea7b039eaad3dc3.

Conflicts:
	testbed/reader-writer/run.sh