utmp: add suppport for FreeBSD

FreeBSD does not have utempter helper but instead have a ulog-helper
which provides the same set of functionnality, but the call and
arguments are slightly different.

doc/foot.ini.5.scd

@@ -344,8 +344,10 @@ empty string to be set, but it must be quoted: *KEY=""*)
 	some cases, the number of physical _cores_ is better.
 
 *utempter*
-	Path to utempter helper binary. Set to *none* to disable utmp
+	Path to _@utempter_name@_ helper binary. Set to *none* to disable utmp
 	records. Default: _@utempter@_.
+	Foot will execute the binary with _@utempter_add@_ when starting
+	and with _@utempter_del@_ when exiting.
 
 # SECTION: environment
 

doc/meson.build

@@ -4,6 +4,7 @@ scdoc_prog = find_program(scdoc.get_variable('scdoc'), native: true)
 
 if utempter_path == ''
   default_utempter_value = 'not set'
+  utempter_name = 'none'
 else
   default_utempter_value = utempter_path
 endif
@@ -12,6 +13,9 @@ conf_data = configuration_data(
   {
     'default_terminfo': get_option('default-terminfo'),
     'utempter': default_utempter_value,
+    'utempter_name': utempter_name,
+    'utempter_add': utempter_add,
+    'utempter_del': utempter_del,
   }
 )