core: merge branch 'th/memleaks'

Fix memleaks and enable valgrind checks for most unit tests except
ifupdown plugin. For ifupdown tests, there are some leaks that are
not yet fixed. This is still to do.

To run checks with valgrind, configure with --with-valgrind.

Especially for libnm and libnm-glib tests, there are several leaks
that are (probably?) not the fault of NetworkManager code. Hence,
several suppressions were added to valgrind.suppressions.
On different systems and different version of glib, these suppressions
might not match and the test will fail there.

The valgrind.suppressions should be reviewed, cleaned up and adjusted
for more systems (and different glib library versions).