Optimize COPYTREE_*

[mat]

Summary

The current chmod commands can't use -exec + because they fiddle with the
pathnames. By switching directory beforehand, it makes it possible to chmod
more than one file at once.

Test Plan

small benchmark, for what it's worth www/sit installs 2k files with a home baked version

	www/sit (2k)	www/mathjax (30k)
home baked	0.032u 0.104s 0:00.13 100.0% 127+236k 4+0io 0pf+0w	0.946u 7.446s 0:08.34 100.4% 40+225k 18276+31io 45pf+0w
current	0.281u 0.649s 0:00.92 100.0% 37+171k 4+0io 0pf+0w	5.446u 18.626s 0:23.99 100.2% 29+171k 18289+31io 0pf+0w
new	0.024u 0.081s 0:00.10 100.0% 177+214k 4+0io 0pf+0w	0.770u 7.012s 0:07.80 99.7% 41+197k 15444+31io 45pf+0w

[mat]

• Add a bit of a comment.

[mat]

This comment was removed by mat.

[mat]

• Collapse two lines, make it more readable.
• Use find -E

Three ports use -regex or -iregex with COPYTREE_*, I'm looking if they need fixing.
devel/ocaml-findlib devel/sdl12 databases/leo_center

[mat]

Good, all three ports using it work just fine with find -E

[mat]

• Think about it and refactor with only one find for all the chmods.