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)
15 jobs for gary-bug-13902-v4-9 in 203 minutes and 14 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #207485280
shared docker
build_ctdb

00:19:17

passed #207485282
shared docker
build_others

00:10:50

passed #207485283
private docker
build_samba

03:23:09

passed #207485286
private docker
build_samba_ad_dc

01:03:17

passed #207485277
shared docker
build_samba_ad_dc_2

00:27:17

passed #207485281
shared docker
build_samba_ctdb

00:35:07

passed #207485285
private docker
build_samba_fileserver

00:16:59

passed #207485278
shared docker
build_samba_libs

00:32:06

passed #207485272
shared docker
build_samba_none_env

00:47:34

passed #207485273
shared docker
build_samba_nopython

00:26:57

passed #207485284
private docker
build_samba_nt4

00:31:45

passed #207485276
shared docker
build_samba_o3

00:34:26

passed #207485279
shared docker
build_samba_static

00:44:00

passed #207485274
shared docker
build_samba_systemkrb5

00:13:18

passed #207485275
shared docker
build_samba_xc

00:04:35