Merge branch 'dev' into 'master'

Improve performances

 * IPv6 support.
 * Zero-copy of packets going through PEPSal (splicing them to their destination using a pipe).
 * Adjustable number of worker threads.
 * More socket options for intercepted connections (CC algorithms, MSS, ssthresh, icwnd, TCP_NODELAY etc.).
 * Fixed issues with file descriptors when using large values of –conns.
 * Fixed deadlock with syslog or fprintf usage in signal handler.
 * Added sniffer thread to parse SYN packets on an interface and open outgoing connections duplicating ToS (IPv4) / Class Field (IPv6) options. The previous version of PEPsal removed the ToS information.
 * Added ability to signal when PEPSal is reaching its connections limit.
 * Improved dependencies detection during compilation.

See merge request openbach/pepsal!8