sd-journal: fix sd_journal_open_directory with SD_JOURNAL_OS_ROOT

sd-journal: fix	sd_journal_open_directory with SD_JOURNAL_OS_ROOT

The directory argument that is given to sd_j_o_d was ignored when
SD_JOURNAL_OS_ROOT was given, and directories relative to the root of the host
file system were used. With that flag, sd_j_o_d should do the same as
sd_j_open_container: use the path as "prefix", i.e. the directory relative to
which everything happens.

Instead of touching sd_j_o_d, journal_new is fixed to do what sd_j_o_c
was doing, and treat the specified path as prefix when SD_JOURNAL_OS_ROOT is
specified.