ignore SIGPIPE

On Solaris, there is no mechanism to disable the delivery of SIGPIPE signals from disconnected sockets – we are making use of such a mechanism on other operating systems. Also, we are installing our dummy SIGPIPE handler using signal(), but the signal handler function seems to assume that it's being installed with sigaction().

We should just ignore SIGPIPEs for all OS's with signal(SIGPIPE, SIG_IGN);. Without this fix, you can crash Solaris mongod rather easily by just generating two socket client disconnects.