sar and pidstat: Check that _("Average") string doesn't exceed

sar and pidstat: Check that _("Average") string doesn't exceed
the size of the timestamp buffer.

One could find something like:
strcpy(string, _("Average"));
in pidstat.c and sar.c. Yet, we don't know whether the translation
message for "Average" will fit in target string buffer. Hence we
replaced the previous expression with something like:
strncpy(string, _("Average"), length_of_string_buffer);
string[lenght_of_string_buffer - 1] = '\0';