Added a code coverage section using gcc and gcov.

  See FILEFORMAT for the description of the test case files.

Code coverage:

  gcc provides a tool that can determine the code coverage figures for

  the test suite.  To use it, configure curl with

  CFLAGS='-fprofile-arcs -ftest-coverage -g -O0'.  Make sure you run the normal

  and torture tests to get more full coverage, i.e. do:

    make test

    cd tests

    make torture-test

  The graphical tool ggcov can be used to browse the source and create

  coverage reports on *NIX hosts:

    ggcov -r lib src

  The text mode tool gcov may also be used, but it doesn't handle object files

  in more than one directory very well.

TEST CASE NUMBERS

 500 - 599   libcurl source code tests, not using the curl command tool

 600 - 699   SCP/SFTP

 700 - 799   SOCKS4 (even numbers) and SOCK5 (odd numbers)

 1000 - x    miscellanous*

 1000 - x    miscellaneous*

 Since 30-apr-2003, there's nothing in the system that requires us to keep

 within these number series, and those sections marked with * actually