dirtree and getallfiles don't check permissions

[RunningDroid]

If dirtree or getallfiles come across a directory they don't have permission to read they throw a stack trace instead of skipping it.

Example:

dir=require('pl.dir')
path=require('pl.path')
path.mkdir('unreadable')
os.execute('chmod -xr unreadable')
return dir.dirtree('unreadable')

the output:

...home/runningdroid/.luarocks/share/lua/5.1/pl/dir.lua:496: cannot open unreadable: Permission denied
stack traceback:
	[C]: in function 'ldir'
	...home/runningdroid/.luarocks/share/lua/5.1/pl/dir.lua:496: in function <...home/runningdroid/.luarocks/share/lua/5.1/pl/dir.lua:488>
	(tail call): ?
	[C]: ?

[Tieske]

Good catch. PR's are most welcome!