cmdmon: convert LOGON timestamp only with LOGON message

Avoid always calling UTI_TimevalNetworkToHost() and make the code more
readable.
util: update functions converting cmdmon timestamps
test: add 115-cmdmontime
rtc: cleanup in error messages
rtc: move preinit call to RTC_Initialise()
rtc: minor cleanup in RTC_Linux_TimePreInit()
rtc: improve accuracy of preinit step
rtc: use LCL functions to read and step system clock
rtc: return status from preinit function
rtc: use fscanf() to read coefficients
rtc: don't try to open rtcfile when not set
rtc: set clock to mtime of driftfile when RTC preinit fails

When the RTC preinit function fails, set the system clock to the time of
the last modification of the driftfile if it's in the future. This makes
the -s option somewhat useful on systems where RTC is not supported or
missing.

This is similar to the functionality implemented in the fake-hwclock
script.
reference: always update driftfile on exit

This is useful with the new fallback function of the -s option to
restore the system time at which chronyd was previously stopped.