Dropped alt-always as an rbyd trunk terminator

Now that tree rebalancing is implemented and needed a null terminator
anyways, I think it's clear that the benefit of the alt-always pointers
as trunk terminator has pretty limited value.

Now a null or other tag is needed for every trunk, which simplifies
checks for end-of-trunk.

Alt-always tags are still emitted for deletes, etc, but there their
behavior is implicit, so no special checks are needed. Alt-always tags
are naturally cleaned up as a part of rbyd pruning.

scripts/dbgmtree.py

@@ -25,7 +25,6 @@ TAG_MTREE       = 0x0306
 TAG_UATTR       = 0x0400
 TAG_SATTR       = 0x0500
 TAG_ALT         = 0x4000
-TAG_ALTA        = 0x6000
 TAG_CRC         = 0x2000
 TAG_FCRC        = 0x2100
 
@@ -173,12 +172,6 @@ def tagrepr(tag, w, size, off=None):
         return 'fcrc%s %d' % (
             ' 0x%x' % w if w > 0 else '',
             size)
-    elif tag == TAG_ALTA:
-        return 'alta w%d %s' % (
-            w,
-            '0x%x' % (0xffffffff & (off-size))
-                if off is not None
-                else '-%d' % off)
     elif tag & 0x4000:
         return 'alt%s%s 0x%x w%d %s' % (
             'r' if tag & 0x1000 else 'b',
@@ -299,7 +292,7 @@ class Rbyd:
                 weight__ += w
 
                 # end of trunk?
-                if not tag & 0x4000 or tag == TAG_ALTA:
+                if not tag & 0x4000:
                     wastrunk = False
                     # update weight
                     weight_ = weight__