vdev_id: use mawk-compatible regular expression

Slot mapping in vdev_id doesn't work on systems using mawk as the 'awk'
alternative. A regular expression in map_slot() contains an unquoted
empty string following the alternation (|) operator, which results in an
"missing operand" error with mawk. The solution is to rearrange the
expression so the alternation has two operands.

Issue zfsonlinux/pkg-zfs#136
Issue zfsonlinux/zfs#2965

Signed-off-by: Ned Bass <bass6@llnl.gov>