[metal] Some minor fixes and tweaks (#933)

* [metal] Ensure DF is clear when calling C from exception handler
* [metal] Mark some internal routines and declarations as `@internal`
* [metal] Fix crash under UEFI when command line string is NULL
* [metal] Fix argc & argv[] setting, & VM page freeing, for UEFI

Part of the memory occupied by the argv[] contents was
erroneously used for page tables & then later erroneously
freed.  The symptom was that argv[0] would show up as an
empty string ("").

libc/vga/tty-graph.c

@@ -30,6 +30,7 @@
 #include "libc/vga/vga.internal.h"
 
 /**
+ * @internal
  * @fileoverview Instantiation of routines for normal console output in
  * graphical video modes.
  *