build-sys: store journald code in a noinst library

      The point is to allow the use of journald functions by other binaries.
Before, journald code was split into multiple files (journald-*.[ch]),
but all those files all required functions from journald.c. And
journald.c has its own main(). Now, it is possible to link against
those functions, e.g. from test binaries.

This constitutes a fix for https://bugzilla.redhat.com/show_bug.cgi?id=872638.

The patch does the following:
1. rename journald.h to journald-server.h and move corresponding code
   to journald-server.c.
2. add journald-server.c and other journald-*.c parts to
   libsystemd-journal-internal.
3. remove journald-syslog.c from test_journal_syslog_SOURCES, since
   it is now contained in libsystemd-journal-internal.
There are no code changes, apart from the removal of a few static's,
to allow function calls between files.