feat: xorg sessions work

leaves a few TODOs:
* error handling
* properly passing VT around
* `wait_for_x_ready`
* allow compile-time overriding `DEFAULT_XORG_DISPLAY` and `XORG_COMMAND`

include/desktop_exec.h

@@ -1,6 +1,9 @@
 #ifndef DESKTOP_EXEC_H_
 #define DESKTOP_EXEC_H_
 
+#include "macros.h"
+
+int execvpe_desktop(char** args, char* NNULLABLE* NNULLABLE envlist);
 int parse_exec_string(const char* exec_s, int* arg_count, char*** args);
 void free_parsed_args(int arg_count, char** args);