Don't block all signals.

Don't block all signals.

Some (buggy?) thread implementations use RT signals internally...
The handling of SIGPIPE remains somewhat problematic. We should probably
use MSG_NOSIGNAL when writing to sockets, but this does not work when
dealing with pipes/FIFOs or writing to sockets using write(). In
particular, net_Write uses write() so that it can be used on non-socket
descriptors too (I believe this is used by the RC interface).