ddb: add the DB_CMD_MEMSAFE flag for commands

This flag value can be used to indicate if a command has the property of
being "memory safe". In this instance, memory safe means that the
command does not allow/enable reads or writes of arbitrary memory,
regardless of the arguments passed to it. For example, 'backtrace' is
considered a memory-safe command since its output is deterministic,
while 'show vnode' is not, since it requires a memory address as an
argument and will print the contents beginning at that location.

Apply the flag to the "show all" command macros. It is expected that
commands added to this table will always exhibit this property.

Reviewed by: markj
Sponsored by: Juniper Networks, Inc.
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D35581