Merge branch 'ps/config-subcommands' into ps/builtin-config-cleanup

* ps/config-subcommands:
  builtin/config: display subcommand help
  builtin/config: introduce "edit" subcommand
  builtin/config: introduce "remove-section" subcommand
  builtin/config: introduce "rename-section" subcommand
  builtin/config: introduce "unset" subcommand
  builtin/config: introduce "set" subcommand
  builtin/config: introduce "get" subcommand
  builtin/config: introduce "list" subcommand
  builtin/config: pull out function to handle `--null`
  builtin/config: pull out function to handle config location
  builtin/config: use `OPT_CMDMODE()` to specify modes
  builtin/config: move "fixed-value" option to correct group
  builtin/config: move option array around
  config: clarify memory ownership when preparing comment strings