Improve WP_Filesystem::dirlist() format consistency

[dd32]

At present the return values of dirlist() between the various methods are scattered between several different formats

Field	Direct	FTP Ext	FTP Sockets	SSH2
name	Yes	Yes	Yes	Yes
perms	Yes	Yes	Yes	Yes
permsn	Yes	Yes	Yes	Yes
number	false	Yes	Yes	false
owner	Yes	Yes	Yes	Yes
group	Yes	Yes	Yes	Yes
size	Yes	Yes	Yes or <DIR>	Yes
lastmodunix	Unix Timestamp	No	No	Unix Timestamp
lastmod	Nov 15	No	No	Nov 15
time	02:12:48	Unix Timestamp	Unix Timestamp	02:12:48
year	No	Maybe	Maybe	No
month	No	Nov	Nov	No
day	No	15	15	No
hour	No	02	02	No
minute	No	12	12	No
type	d or f	d, f, or l	d, f, or l	d or f
isdir	No	Yes	Yes	No
islink	No	Yes	Yes	No

If the FTP Server was Windows, it'd complicate it further by not returning certain fields, using different data in them, or getting confused and not returning any data.

The attached patch boils it down to a consistent result of [ 'filename.ext' => [...], ...] with the nested arrays of:

Field	Value
name	Filename
time	Unix Timestamp of last modified
size	File size or false for Directories
type	d, f, or l for Directory, File, or Link respectively
perms	A formatted human-readable drwxr-xr-x
permsn	A octal as string '0755' for Back-compat purposes
owner	Owner ID or name
group	Group ID or name
files	If it's a recursive directory lookup, a nested array of files

A number of these are only got back-compat or to match the FTP output - most of these fields are not used by WordPress at all - Specifically group, owner, perms, permsn, and time are ignored 100% by WordPress (along with a lot of the methods)
Changing this has some back-compat issues for any plugins doing anything special with the fields - however they've had to work around the fields returning random content already, so this shouldn't be a huge issue (or they were already broken)