Fix unpacking time (unsigned int) from octets for large values

[vanitasvitae]

Time, as well as offsets (e.g. expiration) in OpenPGP are stored as seconds in unsigned Integers.

I noticed, that 8385a2c broke some tests in PGPainless that dealt with large expiration time intervals.

Turns out, the changed code accidentally converted the values to signed ints, which break for large (but legal) intervals.

The patch converts back to an unsigned value.

[vanitasvitae]

I wonder if more places in the code are affected by this, e.g. key creation time...

[vanitasvitae]

PublicKeyPacket and SignaturePacket were also affected by this.

I also discovered a discrepancy between StreamUtils.writeTime() (expecting seconds) and StreamUtils.readTime() (returning millis).

I fixed this by changing writeTime() to expect millis and introducing writeSeconds() and readSeconds() dealing with seconds.

[vanitasvitae]

Okay, now I wonder, if large values are handled properly in general (e.g. large lengths) :D