add get_security_file() function to utils.path
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/ipython,ipython/ipython