Mark default_transaction_read_only as GUC_REPORT.

This allows clients to find out the setting at connection time without
having to expend a query round trip to do so; which is helpful when
trying to identify read/write servers.  (One must also look at
in_hot_standby, but that's already GUC_REPORT, cf bf8a662c.)
Modifying libpq to make use of this will come soon, but I felt it
cleaner to push the server change separately.

Haribabu Kommi, Greg Nancarrow, Vignesh C; reviewed at various times
by Laurenz Albe, Takayuki Tsunakawa, Peter Smith.

Discussion: https://postgr.es/m/CAF3+xM+8-ztOkaV9gHiJ3wfgENTq97QcjXQt+rbFQ6F7oNzt9A@mail.gmail.com

doc/src/sgml/libpq.sgml

@@ -2191,6 +2191,7 @@ const char *PQparameterStatus(const PGconn *conn, const char *paramName);
        <varname>server_encoding</varname>,
        <varname>client_encoding</varname>,
        <varname>application_name</varname>,
+       <varname>default_transaction_read_only</varname>,
        <varname>in_hot_standby</varname>,
        <varname>is_superuser</varname>,
        <varname>session_authorization</varname>,
@@ -2206,7 +2207,8 @@ const char *PQparameterStatus(const PGconn *conn, const char *paramName);
        <varname>IntervalStyle</varname> was not reported by releases before 8.4;
        <varname>application_name</varname> was not reported by releases before
        9.0;
-       <varname>in_hot_standby</varname> was not reported by releases before
+       <varname>default_transaction_read_only</varname> and
+       <varname>in_hot_standby</varname> were not reported by releases before
        14.)
        Note that
        <varname>server_version</varname>,

doc/src/sgml/protocol.sgml

@@ -1278,6 +1278,7 @@ SELCT 1/0;<!-- this typo is intentional -->
     <varname>server_encoding</varname>,
     <varname>client_encoding</varname>,
     <varname>application_name</varname>,
+    <varname>default_transaction_read_only</varname>,
     <varname>in_hot_standby</varname>,
     <varname>is_superuser</varname>,
     <varname>session_authorization</varname>,
@@ -1293,7 +1294,8 @@ SELCT 1/0;<!-- this typo is intentional -->
     <varname>IntervalStyle</varname> was not reported by releases before 8.4;
     <varname>application_name</varname> was not reported by releases before
     9.0;
-    <varname>in_hot_standby</varname> was not reported by releases before
+    <varname>default_transaction_read_only</varname> and
+    <varname>in_hot_standby</varname> were not reported by releases before
     14.)
     Note that
     <varname>server_version</varname>,

src/backend/utils/misc/guc.c

@@ -1619,7 +1619,8 @@ static struct config_bool ConfigureNamesBool[] =
 	{
 		{"default_transaction_read_only", PGC_USERSET, CLIENT_CONN_STATEMENT,
 			gettext_noop("Sets the default read-only status of new transactions."),
-			NULL
+			NULL,
+			GUC_REPORT
 		},
 		&DefaultXactReadOnly,
 		false,