Convert file from dos to unix mode

This is needed in order for vim on Linux (and probably any Unix) to be
able to read the plugin.
Add 'new' option when editing a file to add to a changelist
Recognize p4 and p4.exe as executables
Check that a file is in depot before asking for editing

Before prompting for permission to edit a read-only file, check that the
file is in p4 depot.
If it's not, it's simply a normal RO file in the user's filesystem and
should be treated as such.
If it is, prompt for confirmation and changelist number