tar: extract symlink attributes, close some symlink-related races

tar: extract symlink attributes, close some symlink-related races

* NEWS: Describe symlink-extraction improvements.
* src/extract.c (enum permstatus): Remove.
(fchmod, fchown): Define dummy replacement macros if the system
does not supply them.
(implemented): New function.
(struct delayed_set_stat): Remove members invert_permissions,
permstatus.  They were too confusing, and tried to do too much
in too-little space.  Instead, add members current_mode,
current_mode_mask, interdir, atflag.  All users changed.
(struct delayed_link): Add members mode, atime, mtime, to support
platforms such as BSD where symlinks have these attributes.
All users changed.
(fd_chmod): Renamed from fdchmod.  New argument atflag.  Check for
operation not supported at run-time, not at configure-time.  Put
fd argument first.  All callers changed.
(fd_chown): Likewise, renaming from fdchown.
(fd_stat): Likewise, renaming from fdstat.
(set_mode): Remove args stat_info, cur_info, invert_permissions,
permstatus.  Add args mode, mode_mask, current_mode, current_mode_mask,
atflag.  All callers changed.  Close some races.  Use an easier-to
understand method for computing permissions.  Work around POSIX
incompatibility in Linux fchmodat.  Support extraction of symlink
modes, if the OS allows it.
(set_stat): Remove args cur_info, invert_permissions, permstatus.
Add args current_mode, current_mode_mask, interdir, atflag.
All callers changed.  Close some races.  Support extraction of
attributes on symlinks, if the OS allows it.
(delay_set_stat): Remove args invert_permissions, permstatus.
Add args current_mode, current_mode_mask, mode, atflag.
The ST arg can be null now, indicating that it's an intermediate
directory.  All callers changed.
(repair_delayed_set_stat, apply_nonancestor_delayed_set_stat):
Close some races.
(extract_dir): Also be paranoid if only --same-permissions, due
to semantics of setgid and setuid directories on some hosts.
This closes a race on those hosts.  Simplify calculation of
delay_set_stat arguments; the old code was truly strange and
probably wrong in some border cases.
(extract_dir, extract_file, extract_node, extract_fifo): Don't rely on
unspecified behavior in mode arg of open, mknod, etc.  Instead,
mask out those bits when creating the file, and add them later via
fchmod or chmodat.
(open_output_file): file_name is now const.  Add arg current_mode,
current_mode_mask.  All callers changed.  When overwriting old files,
refuse to overwrite something that is not a regular file, since
we're extracting a regular file.
(extract_file): Remove the FIXME comment.  Whatever the protection
issues were, they should be fixed now.  As a result of all the other
API changes, we now use fchmod etc. rather than chmod etc., closing
some races.
(create_placeholder_file, apply_delayed_links): Record desired
mode and times for symlinks, for OSes that support that.