refclock_phc: open device for writing with extpps option

refclock_phc: open device for writing with extpps option

In version 6.15 the Linux kernel started checking write access on the
PHC file descriptor in the PTP_PIN_SETFUNC and PTP_EXTTS_REQUEST ioctls.
chronyd opened the PHC device as readonly, which caused the PHC refclock
driver configured with the extpps option to fail with the
"Could not enable external PHC timestamping" error message.

To ensure compatibility with new kernel versions, add flags to the
SYS_Linux_OpenPHC() function and open the device with the O_RDWR flag
when the extpps option is enabled.