Fix duplicate SSL_is_dtls in libssl and apps.c

Fix duplicate SSL_is_dtls in libssl and apps.c

Currently, SSL_is_dtls exists in both libssl and apps.c,
and one in libssl is guarded by LIBRESSL_INTERNAL and not exposed yet.
This causes portable build broke with openssl(1) and optionstest.
To solve this temporarily, rename SSL_is_dtls by apps.h.
This temporary renaming will be removed when the SSL_is_dtls() is exposed.

ok jsing@