Add constants to network headers
Modified src/settings.h to match the constants of sys/socket.h
Modified the values of SO_* and SOL_SOCKET to be the same values of glibc
Remove sys/socket.h comment about glibc
Modify NSIG, SIGRTMIN and SIGRTMAX of src/settings.js
Merge pull request #1446 from yukoba/network-constants

Added constants to network headers.