[krbdev.mit.edu #2734] [libdefaults] lifetimes should have a default unit

[Alexandra Ellwood via RT]

Lifetime values set in the libdefaults section should have a default unit.  

We just started supporting the "ticket_lifetime" libdefault.  Since it was 
previously ignored, configuration files with the string 
"ticket_lifetime = 600" and similar values with no units have propogated 
to a large number of configuration files.   People with these configuration 
files will start getting EINVAL from krb5_get_init_creds() when they upgrade 
to 1.4.  

We have already had two reports of people running into this problem with 
1.4-current (UMich and Apple).  I can only assume that it will get worse.

We should either modify the code that reads the libdefault or 
krb5_string_to_deltat().

I do not care if we default to seconds or minutes.  Looking at the config 
files containing this value, people seem to assume it will default to seconds.

Also, we should not return EINVAL from krb5_string_to_deltat in this situation.
How about a "Invalid lifetime" error?  Then at least the user knows to look for
lifetime values.