build: rename openssl configure switches

For consistency's sake, rename:

  --openssl-use-sys
  --openssl-includes
  --openssl-libpath

To:

  --shared-openssl
  --shared-openssl-includes
  --shared-openssl-libpath

And add --shared-openssl-libname while we're at it.

The old switches still work but `./configure --help` won't print them.

Fixes #3591.