core/umount: use libmount to enumerate /proc/swaps

example.swaps with "(deleted)" does not cause bogus entries in the list now,
but a memleak in libmount instead. The memleaks is not very important since
this code is run just once.
Reported as https://github.com/karelzak/util-linux/issues/596.

$ build/test-umount
...
/* test_swap_list("/proc/swaps") */
path=/var/tmp/swap o= f=0x0 try-ro=no dev=0:0
path=/dev/dm-2 o= f=0x0 try-ro=no dev=0:0
/* test_swap_list("/home/zbyszek/src/systemd/test/test-umount/example.swaps") */
path=/some/swapfile o= f=0x0 try-ro=no dev=0:0
path=/dev/dm-2 o= f=0x0 try-ro=no dev=0:0
==26912==
==26912== HEAP SUMMARY:
==26912==     in use at exit: 16 bytes in 1 blocks
==26912==   total heap usage: 1,546 allocs, 1,545 frees, 149,008 bytes allocated
==26912==
==26912== 16 bytes in 1 blocks are definitely lost in loss record 1 of 1
==26912==    at 0x4C31C15: realloc (vg_replace_malloc.c:785)
==26912==    by 0x55C5D8C: _IO_vfscanf (in /usr/lib64/libc-2.26.so)
==26912==    by 0x55D8AEC: vsscanf (in /usr/lib64/libc-2.26.so)
==26912==    by 0x55D25C3: sscanf (in /usr/lib64/libc-2.26.so)
==26912==    by 0x53236D0: mnt_table_parse_stream (in /usr/lib64/libmount.so.1.1.0)
==26912==    by 0x53249B6: mnt_table_parse_file (in /usr/lib64/libmount.so.1.1.0)
==26912==    by 0x10D157: swap_list_get (umount.c:194)
==26912==    by 0x10B06E: test_swap_list (test-umount.c:34)
==26912==    by 0x10B24B: main (test-umount.c:56)
==26912==
==26912== LEAK SUMMARY:
==26912==    definitely lost: 16 bytes in 1 blocks
==26912==    indirectly lost: 0 bytes in 0 blocks
==26912==      possibly lost: 0 bytes in 0 blocks
==26912==    still reachable: 0 bytes in 0 blocks
==26912==         suppressed: 0 bytes in 0 blocks