Intialize 'dt' to 0, this avoids a compiler warning with gcc 4.5 (NetBSD 6.99)

This value is never used though.
Indent the logic to detect the operating system, always include sys/param.h
Add NetBSD as a supported platform.
Only add entries to the leapsecmonths list if their values are legal.
Check that summermonth and wintermonth represent a valid month, set to 0 otherwise.
Note that a value of 0 can be used to disallow DST changes or leap seconds.