journal: use initialization instead of zeroing
journald: do not calculate free space too early

Since the system journal wasn't open yet, available_space() returned 0.

Before:
systemd-journal[22170]: Allowing system journal files to grow to 4.0G.
systemd-journal[22170]: Journal size currently limited to 0B due to SystemKeepFree.

After:
systemd-journal[22178]: Allowing system journal files to grow to 4.0G.
systemd-journal[22178]: Journal size currently limited to 3.0G due to SystemKeepFree.

Also, when failing to write a message, show how much space was needed:
"Failed to write entry (26 items, 260123456 bytes) despite vacuuming, ignoring: ...".
man: add sd_j_open_files to return values section
man: improve readability of "_TRANSPORT=" section in systemd.journal-fields(7)

The list and descriptions of valid transports was difficult to read, so
break the long sentence up into discrete man page list items to improve
readability.
systemd-analyze: Show critical chains for listed units
man: update systemd-analyze invocation