chomp PQerrorMessage() in backend uses

PQerrorMessage() returns an error message with a trailing newline, but
in backend use (dblink, postgres_fdw, libpqwalreceiver), we want to have
the error message without that for emitting via ereport().  To simplify
that, add a function pchomp() that returns a pstrdup'ed string with the
trailing newline characters removed.

src/backend/utils/mmgr/mcxt.c

@@ -1181,3 +1181,17 @@ pnstrdup(const char *in, Size len)
 	out[len] = '\0';
 	return out;
 }
+
+/*
+ * Make copy of string with all trailing newline characters removed.
+ */
+char *
+pchomp(const char *in)
+{
+	size_t		n;
+
+	n = strlen(in);
+	while (n > 0 && in[n - 1] == '\n')
+		n--;
+	return pnstrdup(in, n);
+}