Fixes for resolv.conf detection and specification

[dvershinin]

This PR fixes #1546 and also allows to unconditionally specify location of /etc/resolv.conf, thus avoiding the file check for environments that might not yet have /etc/resolv.conf created; as well as allowing Tengine to have its own app-specific resolv.conf location customized.

E.g.

./configure --with-resolve-file=/etc/resolv.conf

This disables file check during built and unconditionally uses the specified filename.

Summary example:

  ...
  nginx http fastcgi temporary files: "fastcgi_temp"
  nginx http uwsgi temporary files: "uwsgi_temp"
  nginx http scgi temporary files: "scgi_temp"
  resolv.conf file: "/etc/resolv.conf"

When --with-resolve-file is unspecified, /etc/resolv.conf is checked for existence, as prior to the change.

[CLAassistant]

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