[libmultipath] fix the "too many files" error

[libmultipath] fix the "too many files" error

Added a max_fds parameter to /etc/multipath.conf. This allows
you to set the maximum number of open fds that multipathd can use, like with
ulimit -n.  Also added some code so that multipath closes the file descriptor
after it's used by the checker function, since multipath doesn't need to keep
them always open like multipathd does.