find: Add GNU find's -printf

[imp]

Summary

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

NOTE: Man page needs work. Also, this is the perfect case for tests additions, which also haven't been added yet.

Test Plan

Added several people that have touched find in the past few years to get review
Several are docs committers that can help me write the man page.

[bapt]

usr.bin/find/Makefile

15

probably added by accident :D

16

nitpicking here, but maybe consider open_memstream instead of sbuf? it will avoid adding an extra link to sbuf library for a very limited usage. feel free to totally ignore this nitpicking comment

[imp]

use open_memstream

[imp]

usr.bin/find/Makefile

16

Sure. I resisted for a while, but the conversion was quicker than arguing :)

[jrtc27]

usr.bin/find/printf.c

105

\\ needs to be special-cased to not warn

143

[jilles]

usr.bin/find/function.c

1415

We only need to force stat if certain formats are used. If it's just %p/%f/%h and literals, no stat calls are necessary.

usr.bin/find/printf.c

270

This will run off the end of the string if *fmt == '\0'.

282

typo

[gbe]

Usually LGTM, besides the part of the XXX (see comment).

usr.bin/find/find.1

1042

This should be written out or removed. Otherwise LGTM.