libfetch: improve ssl connections

loosely based on the freebsd implementation, implement https
connection settings to override CA, and use client certificate.

new files supported in /etc/apk/:
  ca.pem   - if exists, it contains CAs acceptable for https
	     (otherwise system wide CAs are used)
  crl.pem  - if ca.pem is used, this is the (optional) CRL for it
  cert.pem - used as client authentication certificate (+ key)
  cert.key - used as client key (can be also inside cert.pem)