lib: Determine cpu core count with /proc/cpuinfo.

On systems that provide /proc/cpuinfo similar to Linux on x86, this
should allow us to choose a better default value for the number of
upcall handler threads -- in particular, it avoids counting
hyper-thread cores. If /proc/cpuinfo cannot be parsed for any reason,
fall back to using sysconf().

Signed-off-by: Joe Stringer <joestringer@nicira.com>
Signed-off-by: Ethan Jackson <ethan@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>