Various code fixes

- Resulting from Sonarcloud analysis.

***NO_CI***

Author: josesimoes

System.IO.FileSystem/FileStream.cs

@@ -18,13 +18,13 @@ public class FileStream : Stream
         private bool _canWrite;
         private bool _canSeek;
 
-        private long _seekLimit;
+        private readonly long _seekLimit;
         private long _position;
 
         private bool _disposed;
 
-        private string _name;
-        private string _path;
+        private readonly string _name;
+        private readonly string _path;
 
         #endregion
 

System.IO.FileSystem/Path.cs

@@ -35,10 +35,9 @@ public sealed class Path
         /// <summary>
         /// Provides a platform-specific array of characters that cannot be specified in path string arguments passed to members of the Path class.
         /// </summary>
-        //public static readonly char[] InvalidPathChars = { '/', '\"', '<', '>', '|', ':', '\0', (Char)1, (Char)2, (Char)3, (Char)4, (Char)5, (Char)6, (Char)7, (Char)8, (Char)9, (Char)10, (Char)11, (Char)12, (Char)13, (Char)14, (Char)15, (Char)16, (Char)17, (Char)18, (Char)19, (Char)20, (Char)21, (Char)22, (Char)23, (Char)24, (Char)25, (Char)26, (Char)27, (Char)28, (Char)29, (Char)30, (Char)31 };
         public static readonly char[] InvalidPathChars = { '/', '\"', '<', '>', '|', '\0', (char)1, (char)2, (char)3, (char)4, (char)5, (char)6, (char)7, (char)8, (char)9, (char)10, (char)11, (char)12, (char)13, (char)14, (char)15, (char)16, (char)17, (char)18, (char)19, (char)20, (char)21, (char)22, (char)23, (char)24, (char)25, (char)26, (char)27, (char)28, (char)29, (char)30, (char)31 };
 
-        internal static char[] m_illegalCharacters = { '?', '*' };
+        internal static readonly char[] m_illegalCharacters = { '?', '*' };
 
         #endregion