Fix build with gnutls versions 3.0 to 3.2 (Bug#32446)

Fix build with gnutls versions 3.0 to 3.2 (Bug#32446)

We previously used functions available only in 3.2+ for all 3.x
versions.
* src/gnutls.c [GNUTLS_VERSION_NUMBER < 0x030501]: Replace calls to
gnutls_cipher_get_tag_size with 0.
[GNUTLS_VERSION_NUMBER < 0x030200]: Alias gnutls_cipher_get_iv_size
to gnutls_cipher_get_block_size, gnutls_digest_list to
gnutls_mac_list, and gnutls_digest_get_name to gnutls_mac_get_name.
[WINDOWSNT]: Adjust DLL function definitions and declarations
accordingly.