Fix data loss when acting as filter

This bug was introduced by the recent lseek-related changes.
* src/delete.c (delete_archive_members):
* src/update.c (update_archive):
Copy the member if acting as a filter, rather than lseeking over
it, which is possible if stdin is a regular file.
* src/list.c (skim_file, skim_member):
* src/sparse.c (sparse_skim_file):
New functions, for copying when a filter.
* src/list.c (skip_file): Remove; replaced with skim_file.
All callers changed.
(skip_member): Reimplement in terms of skim_member.
* src/sparse.c (sparse_skip_file):
Remove; replaced with sparse_skim_file.  All callers changed.
* src/update.c (acting_as_filter): New static var.
(update_archive): Set it; this is like delete.c.
* tests/delete01.at (deleting a member after a big one):
* tests/delete02.at (deleting a member from stdin archive):
Also test filter case.

tests/delete01.at

@@ -27,8 +27,10 @@ AT_TAR_CHECK([
 genfile -l 50000 --file file1
 genfile -l 1024 --file file2
 tar cf archive file1 file2
+tar -f - --delete file2 <archive >archout
+tar tf archout
 tar f archive --delete file2
-tar tf archive],
+cmp archive archout],
 [0],
 [file1
 ])

tests/delete02.at

@@ -31,7 +31,9 @@ tar cf archive 1 2 3
 tar tf archive
 cat archive | tar f - --delete 2 > archive2
 echo separator
-tar tf archive2],
+tar tf archive2
+tar f - --delete 2 < archive > archive3
+cmp archive2 archive3],
 [0],
 [1
 2