multipath-tools: Improvement to max_fds

multipath-tools: Improvement to max_fds

Setting max_fds to unlimited doesn't actually work.  In the kernel, there is a
fixed limit to the maximum number of open fds a process can have.  If you try
to set max_fds to greater than this, it fails.  This patch replaces the special
value "unlimited" with a new special value, "max".  If you set max_fds to "max",
multipath will use the actual system limit, which it looks up from
/proc/sys/fs/nr_open.

Signed-off-by: Benjamin Marzinski <bmarzins@redhat.com>