3437. [bug] isc_buffer_init -> isc_buffer_constinit to initialise

3437.   [bug]           isc_buffer_init -> isc_buffer_constinit to initialise
                        buffers with constant data. [RT #32064]

Squashed commit of the following:

commit 3433b96bf11f8c90ccbe412f01d02a6d8bbc2d33
Author: Mark Andrews <marka@isc.org>
Date:   Sat Dec 8 12:41:16 2012 +1100

    isc_buffer_init -> isc_buffer_constinit

commit c22dbcc1122a0a44f7b46068e0ccbc25353a57d5
Author: Mark Andrews <marka@isc.org>
Date:   Sat Dec 8 12:38:39 2012 +1100

    isc_buffer_init -> isc_buffer_constinit

commit 900820416c45c1887d0d22d7a010df60a903bd56
Author: Mark Andrews <marka@isc.org>
Date:   Sat Dec 8 12:24:19 2012 +1100

    remove isc_buffer_reconstinit

commit f815711c17b05f9961786a90b9bae902d3c01494
Author: Mark Andrews <marka@isc.org>
Date:   Wed Dec 5 15:42:57 2012 +1100

    add isc_buffer_constinit