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.

libfdisk/src/utils.c

@@ -153,7 +153,20 @@ char *fdisk_partname(const char *dev, size_t partno)
 	if ((strncmp(dev, _PATH_DEV_BYID, sizeof(_PATH_DEV_BYID) - 1) == 0) ||
 	     strncmp(dev, _PATH_DEV_BYPATH, sizeof(_PATH_DEV_BYPATH) - 1) == 0 ||
 	     strncmp(dev, "/dev/mapper", sizeof("/dev/mapper") - 1) == 0) {
-	       p = "-part";
+		asprintf(&res, "%.*s%zu", w, dev, partno);
+		if (access(res, F_OK) == 0){
+			p = "";
+		} else {
+			/* check for partition seperator "p" */
+			p = "p";
+			free(res);
+			asprintf(&res, "%.*s%s%zu", w, dev, p, partno);
+			if (access(res, F_OK) != 0){
+				/* otherwise, default to "-path" */
+				p = "-part";
+			}
+		}
+		free(res);
 	}
 
 	if (asprintf(&res, "%.*s%s%zu", w, dev, p, partno) <= 0)