pmdalinux: improve help text for network metrics

The following three metrics have a wrong description:
network.interface.in.frame network recv read frames ...
network.interface.in.fifo network recv read fifos ...
network.interface.out.fifo network send fifos ...

All three are actually error counters (see net/core/net-procfs.c:
dev_seq_printf_stats()).

Improve the description of all metrics taken from /proc/net/dev
and refer to the actual structure in the kernel for easier
referencing.
pmdalinux: resolve another case of valgrind/957 vs ioctl

Lukas reports the ifreq structure needs the same treatment as
the ethtool_cmd structure for the rawhide valgrind version.