key/key-io.c: Add low-level `key_mergeline' and `key_extractline' functions.

key/key-io.c: Add low-level `key_mergeline' and `key_extractline' functions.

Python 3 doesn't use C `stdio' streams.  In order to integrate properly,
we need to provide some other way to do I/O of key material.  Introduce
new functions which work in terms of lines in the keyring, which a
Python wrapper can transfer between us and a Python file.