utmp: rewrite utmp logging

This patch generalizes the utmp support, to not only support
libutempter, but also ulog (and in the future, even more interfaces).

* Rename config option main.utempter to main.utmp-helper
* Add meson option -Dutmp-backend=none|libutempter|ulog|auto
* Rename meson option -Ddefault-utempter-path to -Dutmp-default-helper-path
* utmp is no longer detected at compile time, but at runtime instead.

Meson will configure the following pre-processor macros, based on the
selected utmp backend:

* UTMP_ADD - argument to pass to utmp helper when adding a record (starting foot)
* UTMP_DEL - argument to pass to utmp helper when removing a record (exiting foot)
* UTMP_DEL_HAVE_ARGUMENT - if defined, UTMP_DEL expects an extra argument ($WAYLAND_DISPLAY)
* UTMP_DEFAULT_HELPER_PATH - path to the default utmp helper binary

The documentation has been updated to mention which arguments are
passed to the helper binary.

Closes #1314

CHANGELOG.md

@@ -46,6 +46,9 @@
 ### Added
 
 * VT: implemented `XTQMODKEYS` query (`CSI ? Pp m`).
+* Meson option `utmp-backend=none|libutempter|ulog|auto`. The default
+  is `auto`, which will select `libutempter` on Linux, `ulog` on
+  FreeBSD, and `none` for all others.
 
 
 ### Changed
@@ -54,9 +57,17 @@
   `CSI R`. The kitty keyboard protocol originally allowed F3 to be
   encoded as `CSI R`, but this was removed from the specification
   since `CSI R` conflicts with the _”Cursor Position Report”_.
+* `[main].utempter` renamed to `[main].utmp-helper`. The old option
+  name is still recognized, but will log a deprecation warning.
+* Meson option `default-utempter-path` renamed to
+  `utmp-default-helper-path`.
 
 
 ### Deprecated
+
+* `[main].utempter` option.
+
+
 ### Removed
 ### Fixed