[PXEXT] Add PXENV_FILE_EXEC call to PXE extensions API.

This allows pxelinux to execute arbitrary gPXE commands.  This is
remarkably unsafe (not least because some of the commands will assume
full ownership of memory and do nasty things like edit the e820 map
underneath the calling pxelinux), but it does allow access to the
"sanboot" command.

src/include/pxe.h

@@ -63,6 +63,7 @@ union u_PXENV_ANY {
 	struct s_PXENV_FILE_SELECT		file_select;
 	struct s_PXENV_FILE_READ		file_read;
 	struct s_PXENV_GET_FILE_SIZE		get_file_size;
+	struct s_PXENV_FILE_EXEC		file_exec;
 };
 
 typedef union u_PXENV_ANY PXENV_ANY_t;