makesyscalls: permit a range of syscall numbers for UNIMPL

[emaste]

Summary

Some ABIs have large gaps in syscall numbers. Allow gaps to be filled as ranges of UNIMPL, with an entry like:

248-1023        AUE_NULL        UNIMPL  dummy

[jhb]

In general I like the approach. It's easy enough to extend to other keywords in the future if needed as well.

sys/kern/makesyscalls.sh

630 ↗

(On Diff #38638)

I guess it isn't obvious to me that syscall_range[2] would always be valid. Does awk return '0' for out-of-range array access? Hmm, seems like it returns the empty string probably from the manpage. I wonder if it would be cleaner to explicitly set syscall_range[2] to syscall in the 'n ==1' case and then this loop could just be 'while (syscall <= syscall_range[2])'?

[emaste]

Always set syscall_range[2] as suggested by jhb