Introduce libcommon-lgpl for liblttng-ctl

liblttng-ctl is a LGPLv2.1 library should should not use GPLv2 code.
Introduce libcommon-lgpl as a static archive containing only LGPLv2.1
compatible code.

This also removes the dependency from liblttng-ctl to liburcu.

Include some source files in libcommon-lgpl.a which are indirectly needed
by source files required in libcommon-lgpl.a:

- endpoint.cpp,
- lttng-elf.cpp,
- lttng-elf.h.

Include some source files in libcommon-lgpl.a which are only needed to
link the lttng executable:

- domain.cpp,
- spawn-viewer.cpp, spawn-viewer.h.

Introduce the new source file hashtable/seed.cpp to move the
lttng_ht_seed symbol in a source file which does not require
liburcu-cds, so it can be present in libcommon-lgpl. This allows
building compile units which are needed in the lgpl common library which
also contain functions which directly refer to lttng_ht_seed.

Programs and libraries which use libhashtable.la are changed to use
libcommon-gpl.la instead. libhashtable becomes internal to libcommon.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I27d2acb823a6d951692a5da88ce32bbe6bafb072