cib: move disk validation code from CIB daemon to CIB library

This revision moves duplicated functionality in cib/io.c:validate_on_disk_cib()
and retrieveCib() to a new, internally exposed function
lib/cib/cib_file.c:cib_read_and_verify(). The original two functions have been
reimplemented as wrappers for the new function, and a new error code
pcmk_err_cib_corrupt has been added.