linux: use CLOCK_BOOTTIME if available
linux: use CLOCK_BOOTTIME if available

Use CLOCK_BOOTTIME to calculate the system uptime. Fall back to CLOCK_MONOTONIC
if CLOCK_BOOTTIME is not available (all pre-2.6.39 kernels).

The problem with CLOCK_MONOTONIC is that it doesn't increase when the system is
suspended, making the uptime differ from what the uptime(1) tool reports.

urbit/archaeology2,urbit/archaeology2,unknownbrackets/maxcso,unknownbrackets/maxcso,urbit/archaeology2,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,urbit/archaeology2,urbit/archaeology2,unknownbrackets/maxcso,urbit/archaeology2,unknownbrackets/maxcso,urbit/archaeology2,urbit/archaeology2,urbit/archaeology2