sd-journal: add logic to open journal files of a specific OS tree

sd-journal: add logic to open journal files of a specific OS tree

With this change a new flag SD_JOURNAL_OS_ROOT is introduced. If specified
while opening the journal with the per-directory calls (specifically:
sd_journal_open_directory() and sd_journal_open_directory_fd()) the passed
directory is assumed to be the root directory of an OS tree, and the journal
files are searched for in /var/log/journal, /run/log/journal relative to it.

This is useful to allow usage of sd-journal on file descriptors returned by the
OpenRootDirectory() call of machined.