Defer the January 19, 2038 date limit in UFS1 filesystems to February 7, 2106

UFS1 uses a signed 32-bit value for its times. Zero is
January 1, 1970 UTC. Negative values of 32-bit time predate
January 1, 1970 back to December 13, 1901. The maximum positive
value for 32-bit time is on January 19, 2038 (my 84th birthday).
On that date, time will go negative and start registering from
December 13, 1901. Note that this issue only affects UFS1 filesystems
since UFS2 has 64-bit times. This fix changes UFS1 times from
signed to unsigned 32-bit values. With this change it will no longer
be possible to represent time from before January 1, 1970, but it
will accurately track time until February 7, 2106. Hopefully there
will not be any FreeBSD systems using UFS1 still in existence by
that time (and by then I will have been dead long enough that no-one
will know at whom to yell :-).

It is possible that some existing UFS1 systems will have set times
predating January 1, 1970. With this commit they will appear as
later than the current time. This commit checks inode times when
they are read into memory and if they are greater than the current
time resets them to the current time. By default this reset happens
silently, but setting the sysctl vfs.ffs.prttimechgs=1 will cause
console messages to be printed whenever a future time is changed.

Reviewed-by: kib
Tested-by: Peter Holm
MFC-after: 1 week
Differential Revision: https://reviews.freebsd.org/D48472