library: Return -errno not print messages

library: Return -errno not print messages

A library should generally return an error value, rather than
printing to stderr a message. procps_loadavg() had a few things
to change:

It had a global buffer, but we don't call this function over and
over except in tload. It also did had two macros where a plain
fopen() would do the job nicely.

This removed the macro FILE_TO_BUF which was used everywhere in oldlib
but only for loadavg in newlib.

This library change will set us up to fix tload.