libpq: Use strerror_r instead of strerror

Commit 453c468 introduced a use of strerror() into libpq, but that
is not thread-safe.  Fix by using strerror_r() instead.

In passing, update some of the code comments added by 453c468, as
we have learned more about the reason for the change in OpenSSL that
started this.

Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: Discussion: https://postgr.es/m/b6fb018b-f05c-4afd-abd3-318c649faf18@highgo.ca

Author: petere

src/backend/libpq/be-secure-openssl.c

@@ -1465,10 +1465,11 @@ SSLerrmessage(unsigned long ecode)
 		return errreason;
 
 	/*
-	 * In OpenSSL 3.0.0 and later, ERR_reason_error_string randomly refuses to
-	 * map system errno values.  We can cover that shortcoming with this bit
-	 * of code.  Older OpenSSL versions don't have the ERR_SYSTEM_ERROR macro,
-	 * but that's okay because they don't have the shortcoming either.
+	 * In OpenSSL 3.0.0 and later, ERR_reason_error_string does not map system
+	 * errno values anymore.  (See OpenSSL source code for the explanation.)
+	 * We can cover that shortcoming with this bit of code.  Older OpenSSL
+	 * versions don't have the ERR_SYSTEM_ERROR macro, but that's okay because
+	 * they don't have the shortcoming either.
 	 */
 #ifdef ERR_SYSTEM_ERROR
 	if (ERR_SYSTEM_ERROR(ecode))

src/interfaces/libpq/fe-secure-openssl.c

@@ -1759,15 +1759,16 @@ SSLerrmessage(unsigned long ecode)
 #endif
 
 	/*
-	 * In OpenSSL 3.0.0 and later, ERR_reason_error_string randomly refuses to
-	 * map system errno values.  We can cover that shortcoming with this bit
-	 * of code.  Older OpenSSL versions don't have the ERR_SYSTEM_ERROR macro,
-	 * but that's okay because they don't have the shortcoming either.
+	 * In OpenSSL 3.0.0 and later, ERR_reason_error_string does not map system
+	 * errno values anymore.  (See OpenSSL source code for the explanation.)
+	 * We can cover that shortcoming with this bit of code.  Older OpenSSL
+	 * versions don't have the ERR_SYSTEM_ERROR macro, but that's okay because
+	 * they don't have the shortcoming either.
 	 */
 #ifdef ERR_SYSTEM_ERROR
 	if (ERR_SYSTEM_ERROR(ecode))
 	{
-		strlcpy(errbuf, strerror(ERR_GET_REASON(ecode)), SSL_ERR_LEN);
+		strerror_r(ERR_GET_REASON(ecode), errbuf, SSL_ERR_LEN);
 		return errbuf;
 	}
 #endif