libmultipath: allow distributions to change partition_delim default

libmultipath: allow distributions to change partition_delim default

partition_delimiter is UNSET by default, falling back to the kernel
default behavior to use "p" for devices ending in a digit, and ""
for others. However, the udev rules for kpartx have always used the
delimiter "-part". If a map name is changed (e.g. if an alias is
reconfigured), multipathd first sets partition names e.g. to name "mpatha1",
which is later changed to "mpatha-part1" by kpartx during udev rule
processing. That's inefficient and superfluous. However, if a distribution
simply changed DEFAULT_PARTITION_DELIM to "-part", users wouldn't be able
to restore the default behavior any more.

This patch allows to set 'partition_delimiter "/UNSET/"' in multipath.conf
to force the previous default behavior, even if the distribution default is
something other than UNSET. The string "/UNSET/" is not a valid partition
delimiter because filenames can't contain slashes, thus user freedom isn't
constrained by this patch.

Signed-off-by: Martin Wilck <mwilck@suse.com>