Use libtool convenience libraries for common files for lldpd and lldpctl.

Use libtool convenience libraries for common files for lldpd and lldpctl.

Instead of directly linking common files, we use another libtool
convenience library (libcommon.la). We also separate server part of
ctl.c into a separate file to avoid -DCLIENT_ONLY flag which was
causing recompilation of most files.

We also move compatibility objects (like strcpy() function) into
libcommon.la.