coap-server: Add in support for using/testing COAP_PKI_KEY_PEM_BUF
coap-server: Add in support for using/testing COAP_PKI_KEY_PEM_BUF

Add in -m option to enable using COAP_PKI_KEY_PEM_BUF instead of
COAP_PKI_KEY_PEM when defining certificates.  This is done by reading in
the contents of the file into a temporary buffer and the buffer is
passed in instead of the filename to the PKI logic.

authmillenon/libcoap,authmillenon/libcoap,authmillenon/libcoap