libsystemd: new library, refactor sd-daemon.[ch] test code

This commit introduces a bare-bones replacement for libsystemd:

 - Build .so file and add --with-libsystemd to configure
 - Add capabilities support to test/src/serv.c
 - Update tests to account for a Finit built w/o libsystemd support

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

Makefile.am

@@ -1,5 +1,5 @@
 ACLOCAL_AMFLAGS    = -I m4
-SUBDIRS            = man plugins src system test tmpfiles.d
+SUBDIRS            = man plugins src system tmpfiles.d
 doc_DATA           = README.md LICENSE contrib/finit.conf
 EXTRA_DIST         = README.md LICENSE ChangeLog.md contrib/finit.conf
 
@@ -9,6 +9,17 @@ endif
 if DOC
 SUBDIRS           += doc
 endif
+if LIBSYSTEMD
+SUBDIRS           += libsystemd
+endif
+
+if LIBSYSTEMD
+SUBDIRS           += test
+# Explicit dependency to ensure libsystemd is built before test
+test: libsystemd
+else
+SUBDIRS           += test
+endif
 
 if BASH
 bashdir            = $(BASH_DIR)