Add support for creating utmp records

This patch adds support for creating utmp records using the ‘utempter’
helper binary from the ‘libutempter’ package.

* New config option ‘main.utempter’
* New meson command line option, -Ddefault-utempter-path. Defaults to
  auto-detecting the path.

The default value of the new ‘main.utempter’ config option depends on
the meson command line option ‘-Ddefault-utempter-path’.

If ‘main.utempter’ is *not* set to ‘none’, foot will try to execute
the utempter helper binary to create utmp records when a new terminal
is instantiated. The record is removed when the terminal instance is
destroyed.

doc/foot.ini.5.scd

@@ -317,6 +317,10 @@ commented out will usually be installed to */etc/xdg/foot/foot.ini*.
 	(including SMT). Note that this is not always the best value. In
 	some cases, the number of physical _cores_ is better.
 
+*utempter*
+	Path to utempter helper binary. Set to *none* to disable utmp
+	records. Default: _@utempter@_.
+
 # SECTION: environment
 
 This section is used to define environment variables that will be set

doc/meson.build

@@ -2,9 +2,16 @@ sh = find_program('sh', native: true)
 
 scdoc_prog = find_program(scdoc.get_variable('scdoc'), native: true)
 
+if utempter_path == ''
+  default_utempter_value = 'not set'
+else
+  default_utempter_value = utempter_path
+endif
+
 conf_data = configuration_data(
   {
     'default_terminfo': get_option('default-terminfo'),
+    'utempter': default_utempter_value,
   }
 )