Stuff next three nibbles of tv_sec into top of tv_nsec to fix y2038

[nabijaczleweli]

The domain of the usec field is [0, 999999] which is [0, 0x000F423F], which leaves us the top three nibbles.
Thus, change the header format from

SSSSSSSS
UUUUUUUU
LLLLLLLL
sec = 0xSSSSSSSS
usec = 0xUUUUUUUU
len = LLLLLLLL

to

SSSSSSSS
sssUUUUU
LLLLLLLL
sec = 0xsssSSSSSSSS
usec = 0x000UUUUU
len = LLLLLLLL

this is fully-equivalent until y2038, and introduces a small-but-constant "error" to parsers that don't understand this in files recorded after y2038.

I provide samples of files recorded in 2050, 2100, 2150:

$ for f in *.ttyrec; do echo $f; od -txzZ -N32 $f; done
2050.ttyrec
0000000 969b2045 000d873a 0000007f 313f5b1b  >E ..:........[?1<
0000020 1b3d1b68 30323f5b 1b683430 6e3b305d  >h.=.[?2004h.]0;n<
0000040
2100.ttyrec
0000000 f4a7017f 0003e797 0000007f 313f5b1b  >.............[?1<
0000020 1b3d1b68 30323f5b 1b683430 6e3b305d  >h.=.[?2004h.]0;n<
0000040
2150.ttyrec
0000000 52b2e2d6 0016b69c 0000007f 313f5b1b  >...R.........[?1<
0000020 1b3d1b68 30323f5b 1b683430 6e3b305d  >h.=.[?2004h.]0;n<
0000040

2050.ttyrec.gz
2100.ttyrec.gz
2150.ttyrec.gz

Closes #37
Closes: Debian#1094187