freebsd: obtain true uptime through clock_gettime()
freebsd: obtain true uptime through clock_gettime()

Obtain true uptime through clock_gettime() instead of subtracting
'bootime' from 'now'.

PR-URL: https://github.com/libuv/libuv/pull/497
Reviewed-By: Saúl Ibarra Corretgé <8a7991f6950ea189abf159d6ec98d2fcc13dc5f6@gmail.com>

unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso