Change "tracefile" to overwrite by default, use "+" to append

It seems more convenient, even if a bit dangerous, to make overwrite the
default mode.

docs/backends/postgresql.md

@@ -46,7 +46,7 @@ session sql("postgresql://dbname=mydatabase");
 
 The set of parameters used in the connection string for PostgreSQL is the same as accepted by the [PQconnectdb](https://www.postgresql.org/docs/current/libpq-connect.html#LIBPQ-PQCONNECTDB) function from the `libpq` library, however in addition to standard PostgreSQL connection parameters, SOCI PostgreSQL backend accepts a couple of additional ones:
 
-* `tracefile`: if specified, enables tracing all database activity using [PQtrace()](https://www.postgresql.org/docs/current/libpq-control.html#LIBPQ-PQTRACE) into the specified file. Note that this file is appended to if it already exists.
+* `tracefile`: if specified, enables tracing all database activity using [PQtrace()](https://www.postgresql.org/docs/current/libpq-control.html#LIBPQ-PQTRACE) into the file with the given path. Note that this file is overwritten by default, prepend it with a plus sign, i.e. use `tracefile=+/path/to/file`, to append to the file instead.
 * `singlerow` or `singlerows`: if set to `true` or `yes`, enables single-row mode for the session (see below).
 
 #### Single Row Mode

src/backends/postgresql/session.cpp

@@ -188,9 +188,20 @@ void postgresql_session_backend::connect(
         single_row_mode_ = connection_parameters::is_true_value(name, value);
     }
 
-    if (params.extract_option("tracefile", value))
+    if (params.extract_option("tracefile", value) && !value.empty())
     {
-        traceFile_ = fopen(value.c_str(), "a");
+        const char* mode;
+        if (value.front() == '+')
+        {
+            mode = "a";
+            value.erase(0, 1);
+        }
+        else
+        {
+            mode = "w";
+        }
+
+        traceFile_ = fopen(value.c_str(), mode);
         if (!traceFile_)
         {
             std::ostringstream oss;