This change accomplishes two things:

- It add an "unusable" (that is its name, not its state) topo method to the IPMI provider. This method will mark unavailable sensor as unusable to indicate to consumers that they cannot use them.

- It will not return data read from an unavailable IPMI sensor.

1.) Recorded the output of "/usr/lib/fm/fmd/fmtopo -S" on a Supermicro X9 2U.

2.) Run full nightly, used "onu" to create new BE and booted into it.

3.) Recorded the new output of "/usr/lib/fm/fmd/fmtopo -S" on the same machine. The differences are as expected:
--- before.txt  2019-11-05 10:40:24.143900715 +0000

+++ after.txt   2019-11-05 11:24:05.281979308 +0000

@@ -1,5 +1,5 @@

 TIME                 UUID

-Nov 05 10:40:23 591d6fa7-b2a0-ee13-ab35-a08b376cd8b2

+Nov 05 11:24:05 48db20e3-81b7-431c-d266-c3c20ecbf8fa
hc://:product-id=T3300-B1:server-id=zebi-46:chassis-id=TS1502-0076/chassis=0

        Present: true

@@ -31,7 +31,7 @@
hc://:product-id=T3300-B1:server-id=zebi-46:chassis-id=TS1502-0076/chassis=0/fan=1

        Present: true

-       Unusable: false

+       Unusable: true
hc://:product-id=T3300-B1:server-id=zebi-46:chassis-id=TS1502-0076/chassis=0/fan=1?sensor=Fan2

        Present: true

@@ -39,7 +39,7 @@
hc://:product-id=T3300-B1:server-id=zebi-46:chassis-id=TS1502-0076/chassis=0/fan=2

        Present: true

-       Unusable: false

+       Unusable: true
hc://:product-id=T3300-B1:server-id=zebi-46:chassis-id=TS1502-0076/chassis=0/fan=2?sensor=Fan3

        Present: true

@@ -63,7 +63,7 @@
hc://:product-id=T3300-B1:server-id=zebi-46:chassis-id=TS1502-0076/chassis=0/fan=5

        Present: true

-       Unusable: false

+       Unusable: true
hc://:product-id=T3300-B1:server-id=zebi-46:chassis-id=TS1502-0076/chassis=0/fan=5?sensor=Fan6

        Present: true
"ipmitool sensor" confirms that the correct sensors were marked as unusable:
Fan1             | 6256.000   | RPM        | ok    | 340.000   | 408.000   | 476.000   | 17204.000 | 17272.000 | 17340.000

Fan2             | na         | RPM        | na    | 340.000   | 408.000   | 476.000   | 17204.000 | 17272.000 | 17340.000

Fan3             | na         | RPM        | na    | 340.000   | 408.000   | 476.000   | 17204.000 | 17272.000 | 17340.000

Fan4             | 6324.000   | RPM        | ok    | 340.000   | 408.000   | 476.000   | 17204.000 | 17272.000 | 17340.000

Fan5             | 6188.000   | RPM        | ok    | 340.000   | 408.000   | 476.000   | 17204.000 | 17272.000 | 17340.000

Fan6             | na         | RPM        | na    | 340.000   | 408.000   | 476.000   | 17204.000 | 17272.000 | 17340.000