Update freeze.py output to match command's usage

Remove --freeze which appears not to match usage.
Use the command's name in the message produced.
Include a "simple" User-Agent string as well as the JSON

To facilitate detection of pip in scenarios where parsing JSON isn't
easy but simple string matching is, change the User-Agent to be of
the form "pip/{version} {json}" instead of just "{json}".

The contents of the JSON data has not changed.
Merge pull request #2086 from dstufft/simpler-json-prefix

Include a "simple" User-Agent string as well as the JSON
Update functional test of freeze for output change
Merge pull request #2085 from alzeih/patch-1

Update freeze.py output to match command's usage
unpack_url: session only needed for unpack_http_url
vcs: add documentation to VersionControl class
cleanup: use "with open()" to open files
freeze: simplify code and use with open()