Use SecureString where there is data to be cleansed

SecureString has quite a few places it should be used in the user
management code.

Tested: ran set password, test password, and other commands

Change-Id: Ia53bc914d25f7965c3e72d5cf18346e0fa9339b9
Signed-off-by: Vernon Mauery <vernon.mauery@linux.intel.com>
diff --git a/user_channel/user_mgmt.cpp b/user_channel/user_mgmt.cpp
index 182d17d..452be90 100644
--- a/user_channel/user_mgmt.cpp
+++ b/user_channel/user_mgmt.cpp
@@ -751,7 +751,7 @@
 }
 
 Cc UserAccess::setSpecialUserPassword(const std::string& userName,
-                                      const std::string& userPassword)
+                                      const SecureString& userPassword)
 {
     if (pamUpdatePasswd(userName.c_str(), userPassword.c_str()) != PAM_SUCCESS)
     {