Introduce NUT "authconf" file support [#3329]

First PoC from AI, slightly modified in review, following the spec
requested in the GitHub issue. Stepping stone for further work.

Signed-off-by: Jim Klimov <jimklimov+nut@gmail.com>

Author: jimklimov

NEWS.adoc

@@ -99,6 +99,8 @@ https://github.com/networkupstools/nut/milestone/13
       allows to pass the `certfile` argument needed for OpenSSL builds. [#3331]
     * The `libupsclient` (C) and `libnutclient` (C++) API were updated to
       report the ability to check `CERTIDENT` information. [#3331]
+    * Introduced support for "authconf" files to store and convey NUT client
+      authentication details. [issue #3329]
 
  - `upsmon` client updates:
     * Introduced support for `CERTFILE` option, so the client can identify

clients/Makefile.am

@@ -110,7 +110,7 @@ endif HAVE_CXX11
 
 # Optionally deliverable as part of NUT public API:
 if WITH_DEV
- include_HEADERS = upsclient.h
+ include_HEADERS = upsclient.h authconf.h
 if HAVE_CXX11
  include_HEADERS += nutclient.h nutclientmem.h
 else !HAVE_CXX11
@@ -170,7 +170,7 @@ upsstats_cgi_LDADD = $(LDADD_CLIENT) $(top_builddir)/common/libcommonstrjson.la
 # but it needs nut_version.h made before the rest of build,
 # to include it into upsclient.c (without an explicit link,
 # this target is sometimes missed in parallel builds):
-libupsclient_la_SOURCES = upsclient.c upsclient.h
+libupsclient_la_SOURCES = upsclient.c upsclient.h authconf.c authconf.h
 
 # See comments for similar trick in common/Makefile.am for common-nut_version.c
 if BUILDING_IN_TREE