Obtain advisory lock on ZED PID file

ZED uses an advisory lock on its state file to protect against
multiple instances running concurrently.  However, work is planned
to move this state information into the kernel, and ZED will still
need to protect against starting multiple instances.

This commit adds an advisory lock on the PID file to protect against
starting multiple instances.  A lock failure can be overridden with
the "-f" (force) command-line option.  The advisory lock on the state
file is being retained for as long as the state information is stored
in the state file.

Signed-off-by: Chris Dunlap <cdunlap@llnl.gov>
Cleanup struct zed_conf vars in zed_conf_destroy

Reset struct zed_conf file descriptors to -1 after close(),
and pointers to NULL after free().

Signed-off-by: Chris Dunlap <cdunlap@llnl.gov>