Promoted _auxtype to a struct to gain access to additional fields

While creating files there was no possibility to explicitly set the
time or date.
This is necessary for example while copying files since the original
time and date should be kept.
Defined a struct to contain the date and time data

To keep backward compatibility auxtype and the date/time are kept
separated.
A struct similar to the one used in dirent.h is used to access the
create_date and create_time fields.
Comments added
Merge pull request #25 from cybernesto/master

Promoted _auxtype to a struct to gain access to additional fields
Adjusted comment.
Made EXTZP segment optional on GEOS as it is used only by CONIO.
Merge remote-tracking branch 'upstream/master'