sd-device: make sd_device_new_from_path() accept relative path to device node

Even though udevadm accepts relative syspath, previously, udevadm
could not use relative path to device node:
===
$ cd /dev
$ udevadm info sda
Bad argument "sda", expected an absolute path in /dev/ or /sys/ or a unit name: Invalid argument
$ udevadm info /usr/../dev/sda
Unknown device "/usr/../dev/sda": No such device
===
With this change, both the above cases work fine.

Note, still sd_device_new_from_devname() requires absolute path starts
with /dev/, for safety.

test/units/TEST-17-UDEV.10.sh

@@ -103,6 +103,11 @@ udevadm info -e --initialized-nomatch >/dev/null
 # udevadm info -c
 udevadm info -w /sys/class/net/$netdev
 udevadm info --wait-for-initialization=5 /sys/class/net/$netdev
+pushd /dev
+udevadm info null >/dev/null
+udevadm info ./null >/dev/null
+popd
+udevadm info /usr/../dev/null >/dev/null
 udevadm info -h
 
 assert_rc 124 timeout 1 udevadm monitor