Add a dummy statement to the beginning of the pthread_cleanup_pop() macro
to allow a call of the macro to be labelled as in:

label:
  pthread_cleanup_pop();

Reviewed by:	imp
MFC after:	3 days
Fix AHCI ATAPI emulation when backed with /dev/cd0

- remove assumption that the backing file/device had
  512-byte sectors
- fix incorrect iovec size variable that would result
  in a buffer overrun when an o/s issued an i/o request
  with more s/g elements than the blockif api

Reviewed by:	Zhixiang Yu (zxyu.core@gmail.com)
MFC after:	3 days
Add a manpage for the getenv() family of functions in the kernel.