Skip over directory entries with no read permission

[dtolnay]

Some systems have certificates that are intended to be only readable by root. I came across this with the following error from rustls-native-certs:

Error {
    context: "failed to read PEM from file",
    kind: Io {
        inner: Os {
            code: 13,
            kind: PermissionDenied,
            message: "Permission denied",
        },
        path: "/etc/pki/tls/certs/sendmail.pem",
    },
}

In my experience, at least with Linux, it is an expectation that tools that load information from arbitrarily many files in a directory will skip over files lacking read permission for the current user. For example the /etc/profile in Debian systems contains the following logic using [ -r ("True if file is readable by you") to skip unreadable files — this makes it convenient to disable individual scripts temporarily using chmod -r and re-enable using chmod +r.
https://salsa.debian.org/sanvila/base-files/-/blob/master/share/profile

if [ -d /etc/profile.d ]; then
  for i in $(run-parts --list --regex '^[a-zA-Z0-9_][a-zA-Z0-9._-]*\.sh$' /etc/profile.d); do
    if [ -r $i ]; then
      . $i
    fi
  done
  unset i
fi

Another example, fc-cache skips font files without read permission in .local/share/fonts.

This is not something that can be implemented downstream of this crate, because the caller of rustls_native_certs::load_native_certs() cannot tell in general whether a PermissionDenied error in CertificateResult.errors originates from loading SSL_CERT_FILE (which we do not want to ignore) vs SSL_CERT_DIR (this PR ignores), without reimplementing much of rustls-native-certs.