Don't reset .bss on reboots.

Since .data isn't reset on a reboot, it's confusing to reset .bss.
Fixup all places that assumed .bss variables were reset.
diff --git a/src/memmap.c b/src/memmap.c
index f74f4f2..16c57ac 100644
--- a/src/memmap.c
+++ b/src/memmap.c
@@ -142,6 +142,7 @@
         dprintf(1, "No room for e820 map!\n");
         return;
     }
+    e820_count = 0;
     e820_list = (void*)bios_table_cur_addr;
     bios_table_cur_addr += msize;
 }