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

@@ -12744,9 +12744,9 @@ if test "$with_openssl" = yes ; then
 fi
 
 if test "$with_ssl" = openssl ; then
-    # Minimum required OpenSSL version is 1.0.1
+    # Minimum required OpenSSL version is 1.0.2
 
-$as_echo "#define OPENSSL_API_COMPAT 0x10001000L" >>confdefs.h
+$as_echo "#define OPENSSL_API_COMPAT 0x10002000L" >>confdefs.h
 
   if test "$PORTNAME" != "win32"; then
      { $as_echo "$as_me:${as_lineno-$LINENO}: checking for CRYPTO_new_ex_data in -lcrypto" >&5
@@ -12961,15 +12961,13 @@ else
 fi
 
   fi
-  # Functions introduced in OpenSSL 1.0.2. LibreSSL does not have
-  # SSL_CTX_set_cert_cb().
-  for ac_func in X509_get_signature_nid SSL_CTX_set_cert_cb
+  # LibreSSL does not have SSL_CTX_set_cert_cb().
+  for ac_func in SSL_CTX_set_cert_cb
 do :
-  as_ac_var=`$as_echo "ac_cv_func_$ac_func" | $as_tr_sh`
-ac_fn_c_check_func "$LINENO" "$ac_func" "$as_ac_var"
-if eval test \"x\$"$as_ac_var"\" = x"yes"; then :
+  ac_fn_c_check_func "$LINENO" "SSL_CTX_set_cert_cb" "ac_cv_func_SSL_CTX_set_cert_cb"
+if test "x$ac_cv_func_SSL_CTX_set_cert_cb" = xyes; then :
   cat >>confdefs.h <<_ACEOF
-#define `$as_echo "HAVE_$ac_func" | $as_tr_cpp` 1
+#define HAVE_SSL_CTX_SET_CERT_CB 1
 _ACEOF
 
 fi