find: Add GNU find's -printf

Implements most of gnu find's -printf predicate. However, the '#', '-',
'.' and size format modifiers are unimplemented, as are %P, %H, %F, %y,
and %Y formats. Follows what I think it should do based on the info
page, I've not looked at the gnu find code.

Sponsored by: Netflix
Discussed with: des, jilles
Reviewed by: pauamma@gundo.com (man)
Differential Revision: https://reviews.freebsd.org/D38138