Fix parsing the timestamps in 16-bit installers
Rename some variables to not confuse naive parsers

*cough*KDevelop*cough*
Make timestamp parsing ignore the local timezone
Abstract platform-specific time functions
Set timestamps for extracted files

Implements: enhancement #5
Add an option to set a timezone for timestamps

... and to disable setting the timestamps.

See: enhancement #5
Document the --timestamps option in the man page