Revert "move file IO functions from misc2.c to fileio.c"

This reverts commit 19c2ede904590ed32fd5e2f3aafe161a12276a4f.

In reference to PR #1430, the decision was made to, instead of
move functions out of misc2.c, to simply rename misc2.c to core.c.
This revert commit undoes the initial migration of function defs
out of misc2.c in preparation for library name change.
rename misc2.c to core.c

The remaining functions in misc2.c are "fundamental" ones relating
to, for instance, writing bytes and times. At this point, it makes
more sense to simply rename misc2.c to better reflect the "core"
functionality that the functions contained within implement,
hence "core.c".