* include/am_defs.h: use system's hasmntopt() only if it is found

* include/am_defs.h: use system's hasmntopt() only if it is found
and it is not buggy.

* libamu/hasmntopt.c (nextmntopt): isspace() takes an int, not a
char.

* amd/am_ops.c, amd/amfs_auto.c, amd/autil.c, amd/nfs_subr.c,
amd/ops_cdfs.c, amd/ops_nfs.c, amd/srvr_nfs.c, libamu/hasmntopt.c,
libamu/mount_fs.c, libamu/mtab.c: rename all uses of hasmntopt()
to amu_hasmntopt().  Don't use hasmntopt() directly any more!