Because, select(),poll(), read(), write() and many other overloaded functions are
normally reentrant from signal handlers, we need to block all signals while we hold
mutex or else we can deadlock.

Modified Makefile.am because while make did not build libtrickle.so,
'make install' was trying to install it. I'm really shooting in the dark for
autoconf problem so maybe it is not the best way to resolve the problem.

With the help of a set of macros, oset is now an automatic variable
on the stack of each thread instead of an unprotected global var.

The modif compiles but is not tested yet. I will launch an endurance
test and report back every few days how things go.

Need to be explicit on where to find the symbol. When called, the libpthread may or may not be loaded.
I have seen segfaut inside the call to safe_vprintf() from errx() when dlsym fail.