Use socketpair() to create sockets

[jakobrs]

Simplifies sockethelpers.cpp, removes the need to place files in the file system, and might make libTAS less likely to break when running multiple instances at the same time.

Possible cons:

• Might break things if an application closes unrecognised file descriptors for no reason
  • Neither wine, nor gdb does this so it should be fine

Supersedes #420.

[jakobrs]

Want to figure out how to get this to work with lldb, which closes all fds except 0, 1, and 2 by default.