find: add SIGINFO handler

[meka_tilda.center]

Summary

Print number of files processed and path currently being processed on SIGINFO.

[asomers]

It LGTM, but let's get some outside review, too. @des what do you think?

[des]

usr.bin/find/find.c

171

Move up.

usr.bin/find/main.c

67

Rename, this doesn't actually show any info. Also consider moving below main() for consistency.

[des]

usr.bin/find/find.c

216

This is redundant since stderr is usually unbuffered, I suggest but will not insist on removing before committing.