GCC 8.0's -Wcast-function-type objects casting function pointers to ones
with incompatible prototypes. Sometimes we do that on purpose though.

Notably, the g_source_set_callback()'s func argument can point to functions
of various prototypes. Also, libnm-glib/nm-remote-connection is perhaps
just not worth reworking, that would just be a waste of time.

A cast to void(*)(void) avoids the GCC warning, let's use it.

UDev never creates such invalid escape sequences. Anyway,
we cannot accept a NUL character at this point. Just take
the ill escape verbatim -- it should never happen anyway.

This can result in trailing garbage (which fails UTF-8 validation
checks) or even worse, in read-out-of-bounds.

Fixes: 6808bf8195d427975638610781f8c5384228218d

https://bugzilla.redhat.com/show_bug.cgi?id=1443114
https://bugzilla.redhat.com/show_bug.cgi?id=1451160
https://bugzilla.redhat.com/show_bug.cgi?id=1451286

      CC       shared/nm-utils/libnm_core_libnm_core_la-nm-udev-utils.lo
    In file included from ./shared/nm-utils/nm-glib.h:27:0,
                     from ./shared/nm-utils/nm-macros-internal.h:29,
                     from ./shared/nm-default.h:178,
                     from shared/nm-utils/nm-udev-utils.c:21:
    shared/nm-utils/nm-udev-utils.c: In function ‘nm_udev_client_enumerate_new’:
    ./shared/nm-utils/gsystem-local-alloc.h:53:50: error: ‘to_free’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
     GS_DEFINE_CLEANUP_FUNCTION(void*, gs_local_free, g_free)
                                                      ^~~~~~
    shared/nm-utils/nm-udev-utils.c:147:18: note: ‘to_free’ was declared here
        gs_free char *to_free;
                      ^~~~~~~
    In file included from ./shared/nm-utils/nm-glib.h:27:0,
                     from ./shared/nm-utils/nm-macros-internal.h:29,
                     from ./shared/nm-default.h:178,
                     from shared/nm-utils/nm-udev-utils.c:21:
    shared/nm-utils/nm-udev-utils.c: In function ‘nm_udev_client_new’:
    ./shared/nm-utils/gsystem-local-alloc.h:53:50: error: ‘to_free’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
     GS_DEFINE_CLEANUP_FUNCTION(void*, gs_local_free, g_free)
                                                      ^~~~~~
    shared/nm-utils/nm-udev-utils.c:243:20: note: ‘to_free’ was declared here
          gs_free char *to_free;
                        ^~~~~~~

Fixes: e32839838e5ea74ba490cf912e20939afa0e4f40

DRY.

GUdevClient always creates a monitor instance, even if there are no subsystems
or handlers defined. Hence the first iteration of NMUdevClient did that as
well.

I think that can be avoided however. We only need a monitor when there is
a event handler subscribed. Contrary to GUdevClient, we know that from the
very beginning.

libgudev is just a wrapper around libudev. We can
use libudev directly and drop the dependency for
libgudev.