It started with the need of CSA to handle end-of-process (eop) at
do_exit() at exit.c. The hook at exit.c was BSD Accounting specific.

Since the need of Linux system accounting has gone beyond what BSD
accounting provides, i think it is a good idea to create a thin layer
of common code for various accounting packages, such as BSD accounting,
CSA, ELSA, etc. The hook to do_exit() at exit.c was changed to invoke
a routine in the common code which would then invoke those accounting
packages that register to the acct_common to handle do_exit situation.

Here is the description of this acct_common patch:
1) two new files at include/linux/acct_common.h and kernel/acct_common.c
2) A new config flag CONFIG_ACCT_COMMON is created and
    CONFIG_BSD_PROCESS_ACCT and a future CSA config flag depend on it.
    I think it is a good idea to always have acct_common in the kernel;
    in that case, the new config flag may not be really necessary. I can
    go either way.
3) Accounting packages can register themselves to acct_common for
    callbacks. Only do_exit handling is defined now. BSD acct.c has been
    modified to register/unergister to acct_common.
4) The 'enhanced acct data collection' routines have been moved from
    acct.c to acct_common.c. Files used to #include <linux/acct.h> were
    modified to #include <linux/acct_common.h>.

This patch was generated against 2.6.11-rc3-mm2.

Signed-off-by: Jay Lan <jlan@sgi.com>