Code review comment for lp://qastaging/~tyhicks/snap-confine/fixes

r48 addresses the following:

* The write_string_to_file() function doesn't flush the userspace buffer
  before calling fclose() and the return value of fclose() is not checked.
  This means that failed underlying write()s are not noticed.

r49 addresses the following:

* Lines in the seccomp filter file that exceed 80 characters are not handled
  properly in seccomp_load_filters() since it does not verify that a newline or
  EOF has been reached after each call to fgets().

r49 could probably be skipped for the release, if needed, since the seccomp filter is a whitelist and a line longer than 80 chars would just mean that any rule listed on that line would not be added to the whitelist.