rtdmnet module should follow same (un)loading rules as generic module

Added devices/rtdmnet.c, heavily based on devices/generic.c, and with similar purpose. It uses Xenomai's RTNet/RTDM socket instead of Linux socket.

Fix C/C++ linking problem in tool/ethercat

Continued update of master's rtdm part to xenomai 3 : ioctl context now obtained from rtdm_fd

Continued update of master's rtdm part to xenomai 3 : rtdm_user_info_t now replaced by rtdm_fd

migrate posix xenomai example to Xenomai3

migrate native xenomai example to Xenomai3

rt_dev_* has been abandonned in Xenomai3, on user side we use the regular POSIX I/O calls exposed by libcobalt

rtdm skin is equivalent to posix in Xenomai3's xeno-config. Replace XENOMAI_RTDM_*FLAGS by XENOMAI_LIB_*FLAGS to distinguish lib/example flags.

Add module installation path to configure options

Intermediate state while updating master's RTDM part to Xenomai 3

Added --with-xenomai-config to override xenomai config command, also allowing special environment variables to be passed.

Example : --with-xenomai-config="DESTDIR=/path/to/staging /path/to/xeno-config"

Fixed scheduler settings in dc_user example; use CLOCK_MONOTONIC.

Use sched_setscheduler() to set priority in user example.

Applied 0002-use-setup_timer-for-v4.15.patch from Beckhoff.