share/log: change log_syntax from "[a:b] " to "a:b: "

share/log: change log_syntax from "[a:b] " to "a:b: "

Those square brackets don't fit how our other messages look like; we use colons
everywhere else. The "[a:b]" format was originally added in
ed5bcfbe3c3b68e59242c03649eea03a9707d318, and remained unchanged for 7 years,
but in the meantime other conventions evolved.

The new version is also one character shorter.

[/etc/elogind/system/elogind-networkd.service.d/override.conf:2] Failed to parse sec value, ignoring: ...
  ↓
/etc/elogind/system/elogind-networkd.service.d/override.conf:2: Failed to parse sec value, ignoring: ...