systemd-python: export sd_j_get_fd, sd_j_reliable_fd, sd_j_close

sd_journal_get_fd(j) is called j.fileno(), for compatiblity with
Python conventions for file-like objects.

More importantly, those new .seek_head() and .seek_tail() do not
call .get_next(). This is better, if one wants to skip before
retrieving an entry.