winbuild: Allow SSPI build with or without Schannel

The changes introduced in commit 2bfa57bc are not enough
to make it actually possible to use the USE_WINSSL option.
Makefile.vc was not updated and the configuration name which is
used in the build path did not match between both build files.

This patch fixes those issues and introduces the following changes:

- Replaced the -schannel name with -winssl in order to be consistent
with the other options
- Added ENABLE_WINSSL option to winbuild/Makefile.vc (default yes)
- Changed winbuild/MakefileBuild.vc to set USE_WINSSL to true if
USE_SSL is false and USE_WINSSL was not specified as a parameter
- Separated WINSSL handling from SSPI handling to be consistent with
the other options and their corresponding code path