Add crypto_pem_{encode,decode}()

Add crypto_pem_{encode,decode}()

Needed for tls-crypt-v2, but isolated enough to be reviewed as a separate
patch.

The encode API allocates memory, because it fits our typical gc-oriented
code pattern and the caller does not have to do multiple calls or
calculations to determine the required destination buffer size.

The decode API does not allocate memory, because the required destination
buffer is always smaller than the input buffer (so is easy to manage by
the caller) and does not force the caller to use the heap.

Signed-off-by: Steffan Karger <steffan.karger@fox-it.com>
Acked-by: Antonio Quartulli <antonio@openvpn.net>
Message-Id: <20180722100645.5813-1-steffan@karger.me>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg17284.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>