* conf/umount/umount_linux.c (umount_fs): cleanup this function,

* conf/umount/umount_linux.c (umount_fs): cleanup this function,
breaking long "if" statements using "goto out".

* conf/umount/umount_{aix,bsd44,osf,default,linux}.c (umount_fs):
call new utility function umount2_fs() as needed (EBUSY, EIO, or
ESTALE).

* conf/umount/umount_{aix,bsd44,osf,default,linux}.c (umount2_fs):
define a new utility function to invoke forcible/lazy unmounts
without touching any mtab files.  This separate utility function
is useful because it can be called from elsewhere.

* amd/amfs_toplvl.c (amfs_toplvl_init): new function, called
before Amd mounts toplvl mounts, which gives us a hook for cleanup
of a previously dead Amd.  In our case, if the user asked for
forced_unmounts, and the OS supports it, then we try forced/lazy
unmounts on any previous toplvl mounts.  This is useful if a
previous Amd died and left behind toplvl mount points (this Amd
will clean them up!).  WARNING: Don't use forced/lazy unmounts if
you have another valid Amd running, because this code WILL force
those valid toplvl mount points to be detached as well!

* amd/amfs_toplvl.c (amfs_toplvl_umount): don't unconditionally
try forced/lazy unmounts because it will prevent a normal Amd from
terminating and cleaning up properly (self-deadlocking: detached
mounts hang the parent Amd on a stat).  Since we already do
unmounts in the background, then try a safer policy: after trying
the normal unmounts a few times (5 sec), escalate and try forced
unmounts a few times (5 more seconds), and if even that failed,
then try the ultimate -- detached unmounts (which always succeed).
This allows Amd to first try and shutdown cleanly, and gradually
try more forcible ways to shutdown.  On Linux, this procedure will
cleanly shutdown Amd even if there are processes with their CWD on
Amd's mount points (which normally result in EBUSY).

* Makefile.am (EXTRA_DIST_CONF): add new umount_aix.c to distro.

* conf/umount/umount_aix.c: easier if AIX has its own unmount
helper file.

* m4/macros/check_umount_style.m4: AIX needs its own unmount style
file.