Wrap certain POSIX syscalls

[vnlitvinov]

pysamprof uses signals on POSIX to collect stacks at regular intervals.

According to the spec, incoming signal interrupts a blocking wait of certain system calls (like recv).
To make pysamprof less intrusive it needs to probe these system functions and automatically re-launch them if they were interrupted by profiling signal. Note that those functions shouldn't be restarted if they were interrupted by other signals!

Also note that this most likely applies to both Mac and Linux.