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* doc/lispref/files.texi (Changing Files): Document this.
* etc/NEWS: Document this.
* src/fileio.c (realmask): Now a static var, not a local.
(barf_or_query_if_file_exists): New arg KNOWN_TO_EXIST.
Remove arg STATPTR.  All uses changed.
(Fcopy_file): Do not alter permissions of existing destinations,
unless PRESERVE-PERMISSIONS (renamed from
PRESERVE-EXTENDED-ATTRIBUTES) is non-nil.
Avoid race when testing for existing destinations and for
when input and output files are the same.
If changing the group fails, adjust both default and
preserved permissions so that access is not granted to the
wrong group.
(Fset_default_file_modes, init_fileio): Update realmask.
(Fdefault_file_modes): Use realmask instead of calling umask.

Fixes: debbugs:16133