Add CLI flag to opt in to world-readable log files (#3747)

## Issue Addressed

#3732

## Proposed Changes

Add a CLI flag to allow users to opt out of the restrictive permissions of the log files.

## Additional Info

This is not recommended for most users. The log files can contain sensitive information such as validator indices, public keys and API tokens (see #2438). However some users using a multi-user setup may find this helpful if they understand the risks involved.

lighthouse/src/main.rs

@@ -129,6 +129,15 @@ fn main() {
                     to store old logs.")
                 .global(true),
         )
+        .arg(
+            Arg::with_name("logfile-no-restricted-perms")
+                .long("logfile-no-restricted-perms")
+                .help(
+                    "If present, log files will be generated as world-readable meaning they can be read by \
+                    any user on the machine. Note that logs can often contain sensitive information \
+                    about your validator and so this flag should be used with caution.")
+                .global(true),
+        )
         .arg(
             Arg::with_name("log-format")
                 .long("log-format")
@@ -407,6 +416,8 @@ fn run<E: EthSpec>(
 
     let logfile_compress = matches.is_present("logfile-compress");
 
+    let logfile_restricted = !matches.is_present("logfile-no-restricted-perms");
+
     // Construct the path to the log file.
     let mut log_path: Option<PathBuf> = clap_utils::parse_optional(matches, "logfile")?;
     if log_path.is_none() {
@@ -446,6 +457,7 @@ fn run<E: EthSpec>(
         max_log_size: logfile_max_size * 1_024 * 1_024,
         max_log_number: logfile_max_number,
         compression: logfile_compress,
+        is_restricted: logfile_restricted,
     };
 
     let builder = environment_builder.initialize_logger(logger_config.clone())?;