populate file: add insecureLinks

rsync 3.5.0 opens merge files, including per-directory .rsync-filter
files, walking the path one directory at a time, refusing symlinks that
neither root nor the user running rsync owns, and opening each directory
for reading.  It treats the files it can't open that way as missing,
without saying so.  That happens, e.g., below a home directory with mode
711, which the user may enter but not read, or behind another user's
symlink.  Such a source then got synced without its .rsync-filter rules:
excluded files got transferred, and --delete-excluded deleted nothing.

Before syncing with rsync 3.5.0 or later a source containing .rsync-filter
files, populate asks rsync which of them it reads: a dry run with
--debug=FILTER2.  If it reports an existing one as missing, the file
source type's new insecureLinks option decides: with null, the default,
populate fails with an error naming the file; with true, rsync reads such
files anyway (--insecure-links); with false, rsync syncs without them.
Both print a warning.

File scenarios sync a source through a symlink owned by nobody, with each
setting.

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