* conf/nfs_prot/nfs_prot_aix5_2.h: define compatible

* conf/nfs_prot/nfs_prot_aix5_2.h: define compatible
forced-unmount flag.

* conf/umount/umount_default.c (umount_fs): if regular umount got
EBUSY, EIO, or ESTALE, then try forced unmount, if supported.  Try
umount2 (Solaris) or uvmount (AIX).

* conf/umount/umount_{bsd44,osf}.c (umount_fs): if default
umount() failed with EIO or ESTALE, also try forced unmount.

* amd/autil.c (amfs_mount), amd/amfs_toplvl.c
(amfs_toplvl_umount): enable forced/lazy unmounts only if user
asked for it (and dlog it).

* scripts/amd.conf.5: document new force_unmount global parameter.

* doc/am-utils.texi (forced_unmounts Parameter): document new
global parameter.

* amd/conf.c (gopt_forced_unmounts): process forced_unmounts
option (default "no").  Exit with an error if user specified the
option as "yes" but configure couldn't find support for either the
MNT_FORCE or MNT_DETACH flags.

* scripts/amd.conf-sample (forced_unmounts): example usage of new
option.

* amd/amd.h (CFM_FORCED_UNMOUNTS): new flag for forced_unmounts
option.

* m4/macros/header_templates.m4: templates for _DETACH and _FORCE.

* configure.in: check for generic u/mount options "detach" and
"force".

* include/am_utils.h (UMOUNT_FS, umount_fs): new prototypes.
Define AMU_UMOUNT flags for force, detach, and autofs.

* amd/amfs_toplvl.c (amfs_toplvl_umount): pass _FORCE and _DETACH
unmount flags unconditionally.  Pass them to UMOUNT_FS().

* conf/nfs_prot/nfs_prot_linux.h: define MNT_FORCE and MNT_DETACH
if needed, because some Linux systems don't define them
(presumably because it would be too dangerous to expose these
flags to users).

* amd/autil.c (amfs_mount): if mount(2) failed with ESTALE or EIO,
then assume that we're in trouble, possibly because a previous
mount is hung.  So, first try to force a lazy unmount of the old
mount.  If the forced unmount worked, then try again to mount the
desired file system.  If the forced unmount failed, then don't
retry: just return an error.

* amd/amfs_generic.c (amfs_generic_umount), amd/amfs_host.c
(amfs_host_umount), amd/ops_cachefs.c (cachefs_umount),
amd/ops_cdfs.c (cdfs_umount), amd/ops_efs.c (efs_umount),
amd/ops_lofs.c (lofs_umount), amd/ops_nfs.c (nfs_umount),
amd/ops_pcfs.c (pcfs_umount), amd/ops_ufs.c (ufs_umount),
amd/ops_xfs.c (xfs_umount), conf/umount/umount_bsd44.c
(umount_fs), conf/umount/umount_default.c (umount_fs),
conf/umount/umount_linux.c (umount_fs), conf/umount/umount_osf.c
(umount_fs): compute and use unmount specific flags.