server: log errors with fclose(3), remove question on short reads

server: log errors with fclose(3), remove question on short reads

ISO 9899:TC3, section 7.19.8.1 says

  The fread function returns the number of elements successfully
  read, which may be less than nmemb if a read error or end-of-file
  is encountered.

This is not as obvious as it could be, but POSIX 2008 (IEEE Std
1003.1-2008), which we require and intends to be identical to the
C spec on this issue, says

  Upon successful completion, fread() shall return the number of
  elements successfully read which is less than nitems only if a
  read error or end-of-file is encountered.