libfdisk: Fix multipath partition seperators for user-friendly names

libfdisk: Fix multipath partition seperators for user-friendly names

The current code assumes "-part" is the only partition sepereator
but this is not true for some distros.

For example in Ubuntu 18.04 fdisk does not print the correct names for
mpatha:

~# ls -l /dev/mapper/mpatha*
lrwxrwxrwx 1 root root 7 Feb  1 04:39 /dev/mapper/mpatha -> ../dm-0
lrwxrwxrwx 1 root root 7 Feb  1 04:38 /dev/mapper/mpatha1 -> ../dm-4
lrwxrwxrwx 1 root root 7 Feb  1 04:38 /dev/mapper/mpatha2 -> ../dm-5
lrwxrwxrwx 1 root root 7 Feb  1 04:38 /dev/mapper/mpatha3 -> ../dm-6

~# fdisk -l /dev/mapper/mpatha
Device                   Boot     Start        End   Sectors  Size Id Type
/dev/mapper/mpatha-part1           2048  419432447 419430400  200G 83 Linux
/dev/mapper/mpatha-part2      419432448  838862847 419430400  200G 83 Linux
/dev/mapper/mpatha-part3      838862848 1258291199 419428352  200G 83 Linux

Instead of assuming a partition seperator of "-part" this patch uses
access to check the file system for a partition seperator of "p" or
the absense of a partition seperator. If neither of these work the patch
defaults to "-part" like we had before this patch.