Remove unused code from configure.

This pull request removes the following from configure:

* `--disable-debuginfo` (unused)
* `--disable-ipv6` (unused)
* `--disable-gnutls-cert` (replaced with `./tools/genssl gnutls`)
* `--disable-openssl-cert` (replaced with `./tools/genssl openssl`)
* `--rebuild` (unused)
* `ENTRYPOINT` (moved to compat.h)
* `SOMAXCONN_S` (unused)