ftp: add support for paths relative to home directory

Motivation:

Globus client makes requests using paths like '~/', with the tilde
symbol is a common place-holder for the users home directory.

Modification:

When resolving a client-supplied path, interpret paths that start '~/'
as the same path without this prefix and resolved relative to the users
home directory.  If the user has no home directory, this is resolved
relative to the root directory.

Note that '~NAME' is a common place-holder for the home directory of
user NAME.  Implementing support for this is deferred until a client
requires it.

Result:

Globus transfer service shows the user's home directory when first
logged it, rather than failing.

Target: master
Request: 3.0
Request: 2.16
Requires-notes: yes
Requires-book: no
Patch: https://rb.dcache.org/r/10064/
Acked-by: Dmitry Litvintsev

Conflicts:
	modules/dcache-ftp/src/main/java/org/dcache/ftp/door/AbstractFtpDoorV1.java

Conflicts:
	modules/dcache-ftp/src/main/java/org/dcache/ftp/door/AbstractFtpDoorV1.java

modules/dcache-ftp/src/main/java/org/dcache/ftp/door/AbstractFtpDoorV1.java

@@ -754,6 +754,7 @@ private static String buildChecksumList(){
     protected boolean _isUserReadOnly;
     protected FsPath _userRootPath = new FsPath();
     protected FsPath _doorRootPath = new FsPath();
+    protected FsPath _userHomePath = new FsPath();
     protected String _cwd = "/";    // Relative to _doorRootPath
     protected FsPath _filepath; // Absolute filepath to the file to be renamed
     protected String _xferMode = "S";
@@ -1421,16 +1422,14 @@ protected void login(Subject subject)
             }
         }
         FsPath doorRootPath;
-        String cwd;
         if (_root == null) {
             doorRootPath = userRootPath;
-            cwd = userHomePath.toString();
         } else {
             doorRootPath = new FsPath(_root);
             if (userRootPath.startsWith(doorRootPath)) {
-                cwd = doorRootPath.relativize(new FsPath(userRootPath, userHomePath)).toString();
+                userHomePath = doorRootPath.relativize(new FsPath(userRootPath, userHomePath));
             } else if (_absoluteUploadPath != null && _absoluteUploadPath.startsWith(doorRootPath)) {
-                cwd = doorRootPath.relativize(_absoluteUploadPath).toString();
+                userHomePath = doorRootPath.relativize(_absoluteUploadPath);
             } else {
                 throw new PermissionDeniedCacheException("User's files are not visible through this FTP service.");
             }
@@ -1441,11 +1440,12 @@ protected void login(Subject subject)
         _listSource = new ListDirectoryHandler(_pnfs);
 
         _subject = mappedSubject;
-        _cwd = cwd;
+        _cwd = userHomePath.toString();
         _doorRootPath = doorRootPath;
         _userRootPath = userRootPath;
         _isUserReadOnly = isUserReadOnly;
         _identityResolver = _identityResolverFactory.withSubject(mappedSubject);
+        _userHomePath = userHomePath;
     }
 
     public static final String hh_get_door_info = "[-binary]";
@@ -2041,6 +2041,11 @@ public void ftp_prot(String arg)
 
     private FsPath absolutePath(String path) throws FTPCommandException
     {
+        if (path.equals("~")) {
+            path = _userHomePath.toString();
+        } else if (path.startsWith("~/")) {
+            path = _userHomePath + path.substring(1);
+        }
         FsPath relativePath = new FsPath(_cwd);
         relativePath.add(path);
         FsPath absolutePath = new FsPath(_doorRootPath, relativePath);