kconfig: require an exact match for "is not set" to disable CONFIG option

Currently, any string starting "is not set" disables a CONFIG option.

For example, "# CONFIG_FOO is not settled down" is accepted as valid
input, functioning the same as "# CONFIG_FOO is not set". It is a
long-standing oddity.

Check the line against the exact pattern "is not set".

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>