include cleanups: remove unused headers and add feature test macros

src/process/fexecve.c

@@ -1,5 +1,4 @@
 #include <unistd.h>
-#include <stdio.h>
 #include <errno.h>
 
 void __procfdname(char *, unsigned);

src/process/system.c

@@ -1,5 +1,5 @@
 #include <unistd.h>
-#include <fcntl.h>
+#include <stdlib.h>
 #include <signal.h>
 #include <sys/wait.h>
 #include <spawn.h>

src/process/vfork.c

@@ -1,3 +1,4 @@
+#define _GNU_SOURCE
 #include <unistd.h>
 #include "syscall.h"
 #include "libc.h"