[settings] Make fetch_string_setting_copy() easier to use

[settings] Make fetch_string_setting_copy() easier to use

Most callers of functions in the fetch_setting() family treat any
errors as meaning "non-existent setting".  In the case of
fetch_string_setting_copy(), an existent setting can still result in
an error due to memory allocation failure.

Allow the caller to distinguish between a non-existent setting and an
error in allocating memory for the copy, by returning success (and a
NULL buffer pointer) for a non-existent setting.

Signed-off-by: Michael Brown <mcb30@ipxe.org>