- check return of write() as gcc recommends
* apt-pkg/acquire.cc:
  - check return of write() as gcc recommends
* apt-pkg/cdrom.cc:
  - check return of chdir() and link() as gcc recommends
* apt-pkg/clean.cc:
  - check return of chdir() as gcc recommends
* apt-pkg/contrib/netrc.cc:
  - check return of asprintf() as gcc recommends