* lib/mountlist.c: All uses of HAVE_F_FSTYPENAME_IN_STATFS replaced by

* lib/mountlist.c: All uses of HAVE_F_FSTYPENAME_IN_STATFS replaced by
HAVE_STRUCT_STATFS_F_FSTYPENAME.
* m4/fstypename.m4 (gl_FSTYPENAME): Use AC_CHECK_MEMBERS instead
of doing all the work ourselves.
* m4/fsusage.m4 (gl_PREREQ_FSUSAGE_EXTRA): Don't check for
sys/statvfs.h since the code doesn't use HAVE_SYS_STATVFS_H.