build: make NM_MORE_ASSERTS define numeric for different levels of more-asserts

build: make NM_MORE_ASSERTS define numeric for different levels of more-asserts

Allows to enable more-asserts more granularly.

Unfortunately, the old check was "${enable_more_asserts} == "yes", thus
we cannot extend "--enable-more-assert=level" because that would mean
that the same build script cannot set the option on both old and new
NetworkManager.
Thus, add a new option --with-more-asserts=level. If you put the
following in your build script, it will work as expected whether
you build a new or an old version of NetworkManager.
  ./configure --enable-more-asserts --with-more-asserts=5