Add helper script convsrctest.pl to manipulate --libcurl tests.

The intention is to take the output of curl's --libcurl option,
as exercised in test 14xx, and generate a corresponding test15xx
in which the generated code is compiled and run.  This will verify
that the generated code behaves equivalently to the original
invocation of the curl command.

The script is not yet integrated into the configure / makefile
machinery.