open_excl opens the file filename for writing and returns the file
handle. The file may not exist before the call to open_excl. The
file will be created with mode 0600.
If there was an error creating the file, open_excl returns -1
and sets errno accordingly.
Since open_excl relies on the O_EXCL flag to open, it does not work
reliably over NFS (the NFS protocol is broken) and must be emulated
using a lock file (create a file with a unique file name and link(2) it
to the lock file. Then stat the lock file and see if the link count is