[tar] Harden timestamp parsing

[tar] Harden timestamp parsing

Improves the parsing of timestamps in a couple of ways:

* Saturate when timestamps exceed the range of time_t.
  In particular, this provides more rational behavior on
  systems with 32-bit time_t.

* Validate the format of overlong pax timestamps.
  We previously failed to check that high-resolution
  timestamps had only digits in the fractional part.
  We now notice and ignore those with a warning.