config: fixed issues with flags/options set by environment vars.

Some environment variables didn’t behave like their corresponding
command line flags. The affected flags were --stats, --log-level,
--separator, --multi-tread-streams, --rc-addr, --rc-user and --rc-pass.
Example:

    RCLONE_STATS='10s'
    rclone check remote: remote: --progress
    # Expected: rclone check remote: remote: --progress –-stats=10s
    # Actual: rclone check remote: remote: --progress

Remote specific options set by environment variables was overruled by
less specific backend options set by environment variables. Example:

    RCLONE_DRIVE_USE_TRASH='false'
    RCLONE_CONFIG_MYDRIVE_USE_TRASH='true'
    rclone deletefile myDrive:my-test-file
    # Expected: my-test-file is recoverable in the trash folder
    # Actual: my-test-file is permanently deleted (not recoverable)

Backend specific options set by environment variables was overruled by
general backend options set by environment variables. Example:

    RCLONE_SKIP_LINKS='true'
    RCLONE_LOCAL_SKIP_LINKS='false'
    rclone lsd local:
    # Expected result: Warnings when symlinks are skipped
    # Actual result: No warnings when symlinks are skipped
    # That is RCLONE_SKIP_LINKS takes precedence

The above issues have been fixed.

The debug logging (-vv) has been enhanced to show when flags are set by
environment variables.

The documentation has been enhanced with details on the precedence of
configuration options.

See pull request #5341 for more information.

cmd/cmd.go

@@ -539,7 +539,8 @@ func AddBackendFlags() {
 				if opt.IsPassword {
 					help += " (obscured)"
 				}
-				flag := flags.VarPF(pflag.CommandLine, opt, name, opt.ShortOpt, help)
+				flag := pflag.CommandLine.VarPF(opt, name, opt.ShortOpt, help)
+				flags.SetDefaultFromEnv(pflag.CommandLine, name)
 				if _, isBool := opt.Default.(bool); isBool {
 					flag.NoOptDefVal = "true"
 				}