Fix btime handling reading from /proc/stat

Fix btime handling reading from /proc/stat

Function getbtime() currently makes the assumption that btime==0 equals
btime not being present in /proc/stat. This is not quite accurate, as
timestamp 0 is, in fact, also a valid time (Epoch), and /proc/stat may
report it as such.

We introduce a flag to indicate whether btime was found in /proc/stat.
In this way, btime==0 becomes a valid case, provided /proc/stat
actually reports this as the boot time.

procps can still detect the case of btime actually not being reported
by the kernel.

Signed-off-by: Markus Mayer <mmayer@broadcom.com>