all: cleanup close() handling and clarify nm_close()/nm_close_with_error()

Cleanup the handling of close().

First of all, closing an invalid (non-negative) file descriptor (EBADF) is
always a serious bug. We want to catch that. Hence, we should use nm_close()
(or nm_close_with_error()) which asserts against such bugs. Don't ever use
close() directly, to get that additional assertion.

Also, our nm_close() handles EINTR internally and correctly. Recent
POSIX defines that on EINTR the close should be retried. On Linux,
that is never correct. After close() returns, the file descriptor is
always closed (or invalid). nm_close() gets this right, and pretends
that EINTR is a success (without retrying).

The majority of our file descriptors are sockets, etc. That means,
often an error from close isn't something that we want to handle. Adjust
nm_close() to return no error and preserve the caller's errno. That is
the appropriate reaction to error (ignoring it) in most of our cases.

And error from close may mean that there was an IO error (except EINTR
and EBADF). In a few cases, we may want to handle that. For those
cases we have nm_close_with_error().

TL;DR: use almost always nm_close(). Unless you want to handle the error
code, then use nm_close_with_error(). Never use close() directly.

There is much reading on the internet about handling errors of close and
in particular EINTR. See the following links:

https://lwn.net/Articles/576478/
https://askcodes.net/coding/what-to-do-if-a-posix-close-call-fails-
https://www.austingroupbugs.net/view.php?id=529
https://sourceware.org/bugzilla/show_bug.cgi?id=14627
https://news.ycombinator.com/item?id=3363819
https://peps.python.org/pep-0475/

contrib/scripts/checkpatch.pl

@@ -206,6 +206,7 @@ complain ("Define setting properties with _nm_setting_property_define_direct_*()
 complain ("Use nm_g_array_{index,first,last,index_p}() instead of g_array_index(), as it nm_assert()s for valid element size and out-of-bound access") if $line =~ /\bg_array_index\b/;
 complain ("Use spaces instead of tabs") if $line =~ /\t/;
 complain ("Prefer implementing private pointers via _NM_GET_PRIVATE() or _NM_GET_PRIVATE_PTR() (the latter, if the private data has an opqaue pointer in the header file)") if $line =~ /\b(g_type_class_add_private|G_TYPE_INSTANCE_GET_PRIVATE)\b/;
+complain ("Don't use close()/g_close(). Instead, use nm_close() (or nm_close_with_error()).") if $line =~ /\b(close|g_close)\b *\(/;
 complain ("Use nm_memdup() instead of g_memdup(). The latter has a size argument of type guint") if $line =~ /\bg_memdup\b/;
 
 # Further on we process stuff without comments.