Don't let check_disk hang on hanging file systems
check_disk: Fix pthread start routine type

The function pointer passed as third argument to pthread_create(3) must
be of type void *(*)(void *).
check_disk: Remove unused status variable

We didn't actually use the exit status provided by pthread_join(3) for
anything.
check_disk: Seperate declarations from code
configure.ac: Don't let pthread check depend on OS

Don't check the operating system environment to detect the correct
linker flags for using the POSIX thread library.
Cosmetic change: s/THRLIBS/THREADLIBS/

Use a more intuitive variable name.
NEWS: Mention check_disk enhancement

Closes #867.
Closes #1186.
Merge branch 'handle-hanging-nfs'

* handle-hanging-nfs:
  NEWS: Mention check_disk enhancement
  Cosmetic change: s/THRLIBS/THREADLIBS/
  configure.ac: Don't let pthread check depend on OS
  check_disk: Seperate declarations from code
  check_disk: Remove unused status variable
  check_disk: Fix pthread start routine type
  Don't let check_disk hang on hanging file systems