Support kadmin script mode

Support kadmin script mode

Add support for a command and argments to be specified on the kadmin
command line, with script-friendly behavior.  kadmin_startup() now
yields either a request string or a request argv array, and sets
script_mode in the argv array case.  Informational messages now go
through info() and are suppressed if script_mode is set.  Prompts and
warning messages are also suppressed in script mode.  Error messages
indicating a failure now go through error() and set exit_status if
script_mode is set.  The extended com_err() hook is always installed
so that com_err messages go through error() and set exit_status.

getopt() is now invoked with a leading '+' to suppress Gnu getopt
argument reordering behavior, so that invokers don't need to pass "--"
to prevent query options from being treated as kadmin options.
Non-Gnu getopt implementations should harmlessly treat '+' as a valid
flag option, which has no effect as it will reach the same default
label in the switch statement.

ticket: 7991