LINUX 5.6: define time_t and use timespec/timespec64

LINUX 5.6: define time_t and use timespec/timespec64

The time_t type and the structure timeval were removed for use in kernel
space code in Linux commits:
    412c53a680a97cb1ae2c0ab60230e193bee86387
        y2038: remove unused time32 interfaces
    c766d1472c70d25ad475cf56042af1652e792b23
        y2038: hide timeval/timespec/itimerval/itimerspec types

Add an autoconf test for the time_t type.

If time_t is missing, define the time_t type when building the kernel
module.

Change the vattr structure in LINUX/osi_vfs.h to use timespec/timespec64
instead of the timeval structure.

Conditionalize the definition of gettimeofday (needed by rand-fortuna.c) in
crypto/hcrypto/kernel/config.h.  It is unused by the Linux kernel module
and the function uses struct timeval that is no longer available.

Reviewed-on: https://gerrit.openafs.org/14083
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Cheyenne Wills <cwills@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit 78049987aa3e84865e2e7e0f3dd3b54d66258e74)

Change-Id: Iff80c161441356d19b5962956dd524792b7bf629
Reviewed-on: https://gerrit.openafs.org/14095
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: Cheyenne Wills <cwills@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>