Better checking of error return codes from various thread functions. Added

-dT to enable error checking mutexes in the thread library (--with-rtldebug
is _not_ required).

Rev: man/pike.1:1.17
Rev: src/acconfig.h:1.123
Rev: src/aclocal.m4:1.68
Rev: src/configure.in:1.713
Rev: src/main.c:1.171
Rev: src/main.h:1.22
Rev: src/pike_threadlib.h:1.39
Rev: src/threads.c:1.214