Remove -b6 and -old dump formats

Get rid of the code to dump and load -b6 and -old format dump files.
Loading these versions hasn't worked since at least 1.3.

ticket: 7564 (new)
Reorder dump.c

Without changing anything (except to make a few internal functions
static), reorder dump.c to bottom-up order so that forward function
declarations aren't needed.
Refactor dump.c

When dumping, use a common iterator function to unpack the dump_args
structure, unparse and filter the principal name, and convert master
keys.  Add helper functions to dump and load the "octets or -1" format
used for optional binary fields in the current dump format.
Modernize dump.c

Reformat and simplify dump.c code according to current coding
standards.  No functional changes except for some error messages.