Support Windows user accounts as the source of home directory information in twisted.protocols.ftp.SystemFTPRealm

[exarkun]

#2179 introduced (is about to introduce) SystemFTPRealm which consults the system user database to decide where the root of a particular user's FTP access should be.

For various reasons the implementation only supports POSIX. It would be nice to expand it to support Windows as well.

[exarkun]

(In [29908]) Merge ftprealm-custompath-2179-2

Author: therve, exarkun Reviewer: exarkun, screwtape Fixes: #2179 Fixes: #4494 Refs: #4611

Factor the user home directory selection logic into a helper method of FTPRealm and move the rest of the realm logic into a new BaseFTPRealm class. Add a new realm, SystemFTPRealm, which (on POSIX) can select home directories by looking at the system user database.