add get_security_file() function to utils.path

This makes it easier for third party tools to retrieve security files
(e.g. json connection files) by name and [optional] profile name alone, without
knowledge of IPython's directory structure.

For example, whenever ipkernel outputs:

` --existing kernel-12345.json --profile foo`

that file can be found with:

get_security_file('kernel-12345.json', profile='foo')

IPython/utils/path.py

@@ -476,3 +476,34 b' def check_for_old_config(ipython_dir=None):'
   IPython and want to suppress this warning message, set
   `c.InteractiveShellApp.ignore_old_config=True` in the new config.""")
 
+def get_security_file(filename, profile='default'):
+    """Return the absolute path of a security file given by filename and profile
+    
+    This allows users and developers to find security files without
+    knowledge of the IPython directory structure. The search path
+    will be ['.', profile.security_dir]
+    
+    Parameters
+    ----------
+    
+    filename : str
+        The file to be found. If it is passed as an absolute path, it will
+        simply be returned.
+    profile : str [default: 'default']
+        The name of the profile to search.  Leaving this unspecified
+        The file to be found. If it is passed as an absolute path, fname will
+        simply be returned.
+    
+    Returns
+    -------
+    Raises :exc:`IOError` if file not found or returns absolute path to file.
+    """
+    # import here, because profiledir also imports from utils.path
+    from IPython.core.profiledir import ProfileDir
+    try:
+        pd = ProfileDir.find_profile_dir_by_name(get_ipython_dir(), profile)
+    except Exception:
+        # will raise ProfileDirError if no such profile
+        raise IOError("Profile %r not found")
+    return filefind(filename, ['.', pd.security_dir])
+