Add support for devid and phys_path keys in vdev disk labels

Add support for devid and phys_path keys in vdev disk labels

This is foundational work for ZED.

Updates a leaf vdev's persistent device strings on Linux platform

* only applies for a dedicated leaf vdev (aka whole disk)
* updated during pool create|add|attach|import
* used for matching device matching during auto-{online,expand,replace}
* stored in a leaf disk config label (i.e. alongside 'path' NVP)
* can opt-out using env var ZFS_VDEV_DEVID_OPT_OUT=YES

Some examples:

    path: '/dev/sdb1'
    devid: 'scsi-350000394a8ca4fbc-part1'
    phys_path: 'pci-0000:04:00.0-sas-0x50000394a8ca4fbf-lun-0'

    path: '/dev/mapper/mpatha'
    devid: 'dm-uuid-mpath-35000c5006304de3f'

Signed-off-by: Don Brady <don.brady@intel.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #2856
Closes #3978
Closes #4416