Homebrew should not ignore curlrc

Added doctor check for .curlrc rather than silently ignoring it (#13836).

Closes Homebrew/homebrew#15419.

Signed-off-by: Max Howell <mxcl@me.com>

Removed test in doctor where it actually curl'd a file. It's enough to warn if the curlrc exists. I understand people want to remove the warnings, but the point in the doctor is to help diagnose and not to be some ramification of your UNIX system.