Implement new sd_journal_* functions since previous version

[ledbettj]

sd_journal_open_namespace

namespace: option parameter added to Systemd::Journal.new

sd_journal_stream_fd_with_namespace

namespace: option added to Systemd::Journal.log_stream

sd_journal_(restart|enumerate)_fields

Systemd::Journal#fields will return either an Enumerator or invoke a provided block.

sd_journal_has_(runtime|persistent)_files

Implemented Systemd::Journal#(persistent_runtime)_files?