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.

config.h

@@ -319,6 +319,8 @@ struct config {
 
     env_var_list_t env_vars;
 
+    char *utempter_path;
+
     struct {
         enum fcft_scaling_filter fcft_filter;
         bool overflowing_glyphs;