Merge branch 'topic/cred.4'

Merge branch 'topic/cred.4'

* topic/cred.4:
  (Re)Adjust SMTP and POP3: use namespace prefix for support macros
  (UNTESTED!) Support EXTERNAL (via TLS client certificate)
  Add XOAUTH2/OAUTHBEARER (OAuth 2.0 bearer token, RFC 6750) support
  Detach POP3 to net-pop3.h, step II..
  (Now we can) Detach URL stuff to url.h (, so do it)
  Detach lookup stuff to cred-auth.h
  Detach MD5 stuff to cred-md5.h
  Detach .netrc stuff to cred-netrc.h
  Detach socket stuff to net-socket.h
  Detach POP3 to net-pop3.h, step I..
  Tweak SMTP code style, and detach to net-smtp.h
  (BWDIC!) OPT_SOCKETS -> OPT_NET