The "dpms" command refers to VESA Display Power Management
Signaling, a deprecated standard. It's superseded by VESA DPM.
Instead of tying out command name to a particular standard, use the
neutral term "power".
Logic that obtains current DPMS state is put inside the handler.
sway_output from which the current DPMS state will be obtained is selected by the following logic:
* For '-' and '--' the focused output is used;
* For '*' error "Cannot apply toggle to all outputs" is reported;
* For everything else all_output_by_name_or_id() is used.
Fixes#5929.
Patch tested by compiling with `__attribute__ ((format (printf, 2, 3)))`
applied to `cmd_results_new`.
String usage constants have been converted from pointers to arrays when
encountered. General handler format strings were sometimes modified to
include the old input string, especially for unknown command errors.
