Improve file copy (fcopy()) performance by more than an order of magnitude.

Using a 256KB read buffer for copying files (rather than one byte at a time).
Apparently calling fread() is not the same as a bunch of calls to fgetc()
after all. Or maybe it was the many calls to fputc() being replaced with
fwrite(). Or maybe it was both. Anyway, decreased the time to copy a 1GB
file from and to a Samba share over a Gb Ethernet network from 13 minutes
to less than a minute. This matters when sbbs is backing up your data/mail
base and the files are big. The mail base is locked while being backed up
and the longer it takes to back up, the longer the mail base is locked and
no mail can be received, read or sent during that time.
3 jobs for dailybuild_linux-x64 in 10 minutes and 47 seconds (queued for 4 seconds)
Status Name Job ID Coverage
  Build
passed build-sbbs #272

00:09:06

passed build-sexpots #273

00:00:13

passed build-syncterm #274

00:01:28