synce4l on dpll v3

[kubalewski]

Adding support for dpll based synce4l flow. Currently supports only dpll in automatic mode, for manual mode dplls it will not work, this shall be added in the future. Detailed configuration changes available in: "synce4l: add support for dpll subsystem related config items" commit.

v3:

• redesign kernel headers inclusion for compilation on older kernels
• add if_link.h backup header
• fix compilation warning - zero initialize uint64_t u64 in parse_item()

v2:

• redesign behavior of dpll_mon/state machine for easier reinit
• remove async read of synchronous sockets
• fix dpll_rt_recv(..) behavior
• fix crash with one source configured
• netlink fixes/improvements
• reduce cpu usage
• add info logs for external source holdover/locked states
• fix function argument doxygens

[kubalewski]

v4 of this was merged into dev, closing