Suggestion to use more of GCC function attributes

[Marin Ramesa]

We already have noreturn attribute. I have identified two more
attributes that might be useful. These are pure and unused.

[PATCH 1/2] kern/strings.c (strlen): mark with attribute pure

Attribute pure means that a function doesn't have any effects except
the return value, and the return value doesn't depend on return values
from other functions. GCC optimizes functions marked with this attribute.

[PATCH 2/2] ddb/db_command.c (db_help_cmd): mark with attribute unused

Attribute unused means just that, that a function is unused. It might be
useful to identify and mark these functions. They might be scheduled for
removal in future versions.