Stop rewriting an alias file's own metadata

Writing a volume rebuilds every alias file, because the `alis` record
names its target by CNID and CNIDs are assigned afresh on each write.
It rebuilt more than the record, though: the file's type and creator
were overwritten with ones guessed from the target, the rest of the
resource fork was thrown away, and the data fork was cleared.

Round-tripping the Apple Menu's Control Panels alias out of a System 7
image therefore turned `fact` into `fdrp`, dropped its `vers` resources,
and renamed the `alis` from "Control Panels alias" to "Control Panels".
Aliases to applications lost their `adrp` for whatever the target's type
happened to be -- `APPC`, `cdev`.

Only the `alis` resource is replaced now, in place and under its own
name; type and creator are guessed only for an alias built here, which
has neither. Reading and rewriting the 29 infinite-mac images machfs
can parse touches 73 alias files: type and creator used to change on 10
of them and now change on none, and 28 resource forks used to lose more
than the `alis` record and now lose nothing else. (Their data forks were
already empty, so clearing those had cost nothing in practice.)

For mihaip/infinite-mac#84

test_all.py

@@ -1,5 +1,7 @@
 from machfs import *
 from machfs import bitmanip, btree
+from machfs.main import FinderFlags
+from macresources import Resource, make_file, parse_file
 import os
 import struct
 import time
@@ -27,6 +29,38 @@ def test_roundtrip():
     assert f.data in copies[-1]
 
 
+def test_alias_keeps_what_belongs_to_the_file():
+    h = Volume()
+    h.name = 'AliasTest'
+
+    target = File()
+    target.type, target.creator = b'APPC', b'wnkl'
+    target.data = b'target data'
+    h['target'] = target
+
+    # Neither 'fact' nor the 'vers' resource can be derived from the target.
+    vers = Resource(b'vers', 2, data=b'\x01\x00\x80\x00\x00\x00\x041.0')
+    stale = Resource(b'alis', 0, name='alias resource name', data=bytes(150))
+    alias = File()
+    alias.flags |= FinderFlags.kIsAlias
+    alias.type, alias.creator = b'fact', b'MACS'
+    alias.rsrc = make_file([stale, vers])
+    alias.aliastarget = target
+    h['the alias'] = alias
+
+    copy = Volume()
+    copy.read(h.write(10*1024*1024, desktopdb=False, bootable=False))
+    got = copy['the alias']
+
+    assert (got.type, got.creator) == (b'fact', b'MACS')
+
+    resources = {(r.type, r.id): r for r in parse_file(got.rsrc)}
+    assert resources[b'vers', 2].data == vers.data
+    assert resources[b'alis', 0].name == 'alias resource name'
+    # The record itself holds this volume's CNIDs, so it is rebuilt.
+    assert resources[b'alis', 0].data != stale.data
+
+
 def test_open_folder():
     h = Volume()
     h.name = 'OpenFolderTest'