Remove support for OpenSSL 1.0.1

Here are some notes about this change:
- As X509_get_signature_nid() should always exist (OpenSSL and
LibreSSL), hence HAVE_X509_GET_SIGNATURE_NID is now gone.
- OPENSSL_API_COMPAT is bumped to 0x10002000L.
- One comment related to 1.0.1e introduced by 74242c2 is removed.

Upstream OpenSSL still provides long-term support for 1.0.2 in a closed
fashion, so removing it is out of scope for a few years, at least.

Reviewed-by: Jacob Champion, Daniel Gustafsson
Discussion: https://postgr.es/m/ZG3JNursG69dz1lr@paquier.xyz

configure.ac

@@ -1367,8 +1367,8 @@ fi
 
 if test "$with_ssl" = openssl ; then
   dnl Order matters!
-  # Minimum required OpenSSL version is 1.0.1
-  AC_DEFINE(OPENSSL_API_COMPAT, [0x10001000L],
+  # Minimum required OpenSSL version is 1.0.2
+  AC_DEFINE(OPENSSL_API_COMPAT, [0x10002000L],
             [Define to the OpenSSL API version in use. This avoids deprecation warnings from newer OpenSSL versions.])
   if test "$PORTNAME" != "win32"; then
      AC_CHECK_LIB(crypto, CRYPTO_new_ex_data, [], [AC_MSG_ERROR([library 'crypto' is required for OpenSSL])])
@@ -1377,9 +1377,9 @@ if test "$with_ssl" = openssl ; then
      AC_SEARCH_LIBS(CRYPTO_new_ex_data, [eay32 crypto], [], [AC_MSG_ERROR([library 'eay32' or 'crypto' is required for OpenSSL])])
      AC_SEARCH_LIBS(SSL_new, [ssleay32 ssl], [], [AC_MSG_ERROR([library 'ssleay32' or 'ssl' is required for OpenSSL])])
   fi
-  # Functions introduced in OpenSSL 1.0.2. LibreSSL does not have
+  # Function introduced in OpenSSL 1.0.2. LibreSSL does not have
   # SSL_CTX_set_cert_cb().
-  AC_CHECK_FUNCS([X509_get_signature_nid SSL_CTX_set_cert_cb])
+  AC_CHECK_FUNCS([SSL_CTX_set_cert_cb])
   # Functions introduced in OpenSSL 1.1.0. We used to check for
   # OPENSSL_VERSION_NUMBER, but that didn't work with 1.1.0, because LibreSSL
   # defines OPENSSL_VERSION_NUMBER to claim version 2.0.0, even though it