Split YAZ library into two libs : libyaz.la and libyaz_server.la.

Split YAZ library into two libs : libyaz.la and libyaz_server.la.
libyaz.la is the core of YAZ except the generic frontend server and
does not depend on POSIX threads anymore. libyaz_server.la is the
generic frontend server facilities and uses POSIX thread functionality
if available. The libyaz.la no longer depends on POSIX threads because
the number of global structures is limited. NMEM no longer re-uses blocks
between threads ; it simply free's memory immediately but allocates in
"large" chunks as usual. We don't expect any performance penalties
because of this. The yaz_log system is still using a global log_level so
caution must be taken when modifying it with yaz_log_init_.. +
yaz_log_mask_str. This, however, should not cause any trouble because
these functions are called during initialization of application code
anyway. `yaz-config --libs  server` returns libs for server applications;
`yaz-config --libs` returns libs for non-server applications.