net/glib-networking: make GnuTLS and libproxy support optional

• The OpenSSL backend is provided for systems where licensing considerations prohibit use of certain GnuTLS' dependencies
• The environment variable proxy backend is intended for use when building without libproxy support
• Drop long unused dependency on security/p11-kit' which was there for gnutls-pkcs11' backend added in version 2.31.2, disabled (deprecated) in version 2.57.2, and finally removed in 2.59.1 (November 11, 2018) due to lack of any feedback whatsoever regarding its disablement