Avoid some tiny /proc file reads

Avoid some tiny /proc file reads

* src/fileio.c (union read_non_regular):
New members buf and bufsize replace inserted and trytry.
(read_non_regular): Adjust to this new, simpler interface.
(Finsert_file_contents): If the gap is smaller than read_buf
and we want to read more than the gap, read into read_buf
first, to avoid lots of tiny reads from /proc files.

(cherry picked from commit c3f96d20eecdf0aa93f294a4f3fb96e16eec6a68)