Improve file link matching.

Improve file link matching.

Michael Ekstrand wrote:

    I've been trying to work with links to man pages in
    Org (using org-man.el).  When I try to create a link to a
    Perl module in the File package namespace, however,
    `org-insert-link' tries to do its same-file link detection
    and winds up breaking the link.  The result is that the link

       man:File::MimeInfo

    gets rewritten to

       file::MimeInfo

    Naturally, this isn't what I want.  Looking at
    `org-insert-link', it seems that the problem may be that its
    same-file link detection is matching 'file:' at
    beginning-of-word, which it is in this case.  In the general
    case for which 'file:' detection is being used, is there a
    reason to match against `\<' rather than `^' or
    `^[[:space:]]*'?  Changing to one of the latter two
    expressions would, I believe, keep it From rewriting these
    links I am trying to create.

Michael is right, and I have made this change.