core: use non-recursive build

Although having different parts of NM in different subdirectories
keeps the source tree neat, it has made the build messy, particularly
because of cross-dependencies between the subdirs.

Reorganize to build all of the pieces of the NetworkManager binary
from src/Makefile, and only use recursive make for test programs,
helper binaries, and plugins.

As part of this, get rid of all the per-directory convenience
libraries, and switch to building a single top-level
libNetworkManager.la, containing everything except main.c, which all
of the test programs can then link against.

src/posix-signals/Makefile.am

@@ -1,14 +0,0 @@
-noinst_LTLIBRARIES = libnm-posix-signals.la
-
-libnm_posix_signals_la_SOURCES = \
-	nm-posix-signals.c \
-	nm-posix-signals.h
-
-libnm_posix_signals_la_CPPFLAGS = \
-	$(GLIB_CFLAGS) \
-	-DLIBEXECDIR=\"$(libexecdir)\"
-
-libnm_posix_signals_la_LIBADD = \
-	-ldl \
-	$(GLIB_LIBS)
-