Use strtoull when available to parse network metrics on Linux

Not sure if Bugzilla is still used, but I filed a bug over there: http://bugzilla.ganglia.info/cgi-bin/bugzilla/show_bug.cgi?id=342

> In monitor-core/libmetrics/linux/metrics.c, strtoul is used to parse the network packet and byte counts from /proc/net/dev. On modern 32-bit systems (e.g. ARMv7), these counts are often 64-bit, whereas unsigned long is 32 bits. When the counts exceed ULONG_MAX, Ganglia incorrectly reports 0 network bytes in/out from then on.

This commit uses strtoull when it is available for parsing network stats.