fork(2): Add information about fork() in multi-threaded processes.

There is nothing about pthread_atfork(3) or extensions like calling
malloc(3) in the child process as this may be unreliable or broken.
Style cleanups.  No intended functional changes.

 o This driver is the "xbd" driver, not the "blkfront", "blkif", "xbf", or
   "xb" driver.  Use the "xbd_" naming conventions for all functions,
   structures, and constants.
 o The prevailing convention for structure fields in this driver is to
   prefix them with an abreviation of the structure type.  Update
   "recently added" fields to match this style.
 o Remove unused data structures.
 o Remove superfluous casts.
 o Make a pass over the whole driver and bring it closer to
   style(9) conformance.

Sponsored by:	Spectra Logic Corporation
MFC after:	1 week