[host][linux] Change to use unix.SysInfo_t to get Uptime in Linux

[shirou]

According to this and this comments, change to use unix.Sysinfo_t in Linux.

Unfortunately, other platform such as FreeBSD do not include "Uptime" in Sysinfo. So this only for Linux.

[Lomanic]

Couldn't we base BootTime against Uptime like on the other platforms? This would improve performance I think (see #842 (comment)).

[shirou]

Other platform uses time.Now().Unix() via timeSince() function. From my understanding, time.Now() has a monotonic clock but monotonic clock can work only against time object which has a monotonic clock. if not, fall back to wall clock.

Other platform get bootime as just a uint64 which does not have a monotonic clock, so ret value of timeSince() is also not a monotonic. This means, other platform has a problem same as #837 (but not confirmed....).

At least FreeBSD, perhaps we'd better to implement clock_gettime(CLOCK_UPTIME like w source code.

[Lomanic]

In fact we're already using the same thing as timeSince() in host.BootTime() on docker/lxc guests

gopsutil/internal/common/common_linux.go

Line 102 in 6c9f3bd

t := uint64(time.Now().Unix()) - uint64(b)

For the time being I'm merging this, it's already an improvement. 👍