Split getuserdat() into 3 functions:

- openuserdat()
- readuserdat()
- parseuserdat()

getuserdat() still exists and just calls those 3 functions. This allows other
code (e.g. the control panel) to optimize iterative calls to getuserdat() by
opening the file just once or even reading the entire contents at once and then
parsing each user record from memory.

Updated style of arguments in userdat.h (i.e. remove argument name if obvious
from the type).