Cleanup includes

Improved manually with the help of neovim LSP warnings and iwyu:

    iwyu -Ibuilddir/app/ -Iapp/src/ app/src/XXX.c

app/src/sys/unix/file.c

@@ -1,10 +1,11 @@
 #include "util/file.h"
 
 #include <limits.h>
-#include <stdlib.h>
 #include <stdio.h>
+#include <stdlib.h>
 #include <string.h>
 #include <sys/stat.h>
+#include <sys/types.h>
 #include <unistd.h>
 #ifdef __APPLE__
 # include <mach-o/dyld.h> // for _NSGetExecutablePath()

app/src/sys/unix/process.c

@@ -4,6 +4,8 @@
 #include <errno.h>
 #include <fcntl.h>
 #include <signal.h>
+#include <stdio.h>
+#include <stdlib.h>
 #include <sys/types.h>
 #include <sys/wait.h>
 #include <unistd.h>