lib util debug: Increase format buffer to 4KiB

Increase the debug line buffer to 4KiB, the existing size of 1KiB is too
small for dsdbChange JSON audit messages.  These messages were then
split across multipe lines causing issues in log ingestion tools
expecting single line messages.

Bug: https://bugzilla.samba.org/show_bug.cgi?id=13902

Signed-off-by: Gary Lockyer <gary@catalyst.net.nz>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>

Autobuild-User(master): Gary Lockyer <gary@samba.org>
Autobuild-Date(master): Mon May  6 01:18:22 UTC 2019 on sn-devel-184

(cherry picked from commit ad3af7cd)
22 jobs for gary-bug-13902-v4-10 in 206 minutes and 37 seconds (queued for 1 second)
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