Problem: in the near future we would like to create a PlugList
from a List of Plugs.  It is currently inconvenient to create
such a list given the current PlugList API.

Add a function pluglist_copy_from_list() function to create a PlugList
from a List of Plugs.

Problem: In powerman device files the only "interpretation" that
is done is the output of a power status query via the
"setplugstate" statement.  Other interpretations may be done
in the future against other power operations.  Internally anything
associated with "setplugstate" is called "interpretation" (or
"interp" or "interps", etc.).  This will get confusing with future
interpretations.

Solution:  Rename any "interpreations" related code associated
with "setplugstate" to "state interp" (i.e. "StateInterp" or
"state_interp", etc.).

Problem: In many cases, there is no way for a power operation (i.e.
power on, but not power status) to inform powerman that an error has
occurred in the operation.  The user will always get a
"Command completed successfully" output and exit status 0 after
issuing a power operation.

Solution: Support a new "setresult" statement that can inform powerman
that a power operation did not succeed.  A regex can be used to determine
what output is expected of a successful power operation.  If any are not
successful, powerman can subsequently inform the user an error has occurred,
leading to a "Command completed with errors" message and exit status 1.

Some example uses:

script on_all {
	send "on\n"
	foreachnode {
		expect "([^\n:]+): ([^\n]+\n)"
		setresult $1 $2 success="^ok\n"
	}
	expect "redfishpower> "
}

script on {
	send "on %s\n"
	expect "([^\n:]+): ([^\n]+\n)"
	setresult $1 $2 success="^ok\n"
	expect "redfishpower> "
}

Fixes chaos#79

Problem: The new setresult statement is not documented ni powerman.dev(5)

Add information about the new device statement.

Problem: The new setresult statement is not covered in the testsuite.

Add tests in a new t0034-power-result.t.  Add new test device files
etc/redfishpower-setresult-all.dev, etc/redfishpower-setresult-range.dev,
and etc/redfishpower-setresult-singlet.dev which cover the "all", "range",
and "singlet" versions of various power operations (i.e. on_all, vs
on_range, vs on).

Problem: When power control/query to a target fails, there is no way for
a user to know why it failed.

Add a new --diag to powerman that will inform powermand to send
diagnostic information about why a power operation failed.
This only works with setplugstate and the new setresult statement.

Problem: There is no coverage for the new --diag option.

Add tests in new t0035-diag.t tests.

Problem: The new --diag option is not documented.

Add it in powerman(1).