netrc: don't ignore the login name specified with "--user"

- for "--netrc", don't ignore the login/password specified with "--user",
  only ignore the login/password in the URL.
  This restores the netrc behaviour of curl 7.61.1 and earlier.
- fix the documentation of CURL_NETRC_REQUIRED
- improve the detection of login/password changes when reading .netrc
- don't read .netrc if both login and password are already set

Fixes #3213
Closes #3224