[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/intrin/interrupts.S

@@ -92,6 +92,7 @@ __excep0_isr:
 	mov	%eax,%ss
 	mov	%eax,%ds
 	mov	%eax,%es
+	cld				# make sure DF is reset, for C code
 	ezlea	.excep_msg,di		# stack should be 16-byte aligned now
 	xor	%eax,%eax		# kprintf is variadic, remember to
 					# pass no. of vector regs. used (= 0)