Simple plugable VFS

[andrew_b]

Currently, the each external VFS type must be registered in extfs.ini file. It difficult to use third-party plug-ins. I propose to use extfs.ini.d directory instead of extfs.ini file. This directory will contain unzup, rpm, deb, a:, dpkg: etc. files. Each file can be empty or contain some comments. If some filesystems cannot contain files with colon symbol in name, we can use 'a+' or another filename template instead of 'a:'.