Move PortState definition to ipcdef.hpp

Applications that want to make use of the shared memory provided by daemon_cl
should only need to include ipcdef.hpp and ptptypes.hpp (only these two would
have to be installed in /usr/include)
gptp_daemon: feature - Add test for SIGHUP to trigger saving restore data
Move PPS stop to the end

On embedded systems, the file systems containing the configuration file might
not be mounted writable anymore by the time the ptp daemon is killed during
shutdown. With this extension, a SIGHUP can be sent to the ptp daemon in
advance to trigger saving the current configuration at any time.
Temporarily change the umask of gptp daemon to allow group access to shmem

Applications that want to make use of the shared memory provided by gptp daemon
might not run with the same user id. Change the umask of the daemon to allow
access at group level.
Various minor bug fixes:

* Slave does not adopt absolute time (epoch) from the GM when being started
as “always slave” with prio1=255
* Some bugs in the logic of “timestamp versioning”, i.e. avoiding to compare
timestamps from before and after an epoch change