[FTP] add EPRT support
[FTP] add EPRT support

The code does not support some corner cases that are listed in the RFC
(see inline comments), but I suspect those will never be hit. We can add
those when they are hit.

xkahn/gvfs-cmis,gicmo/gvfs,thiblahute/gvfs,gicmo/gvfs,gicmo/gvfs,halfline/gvfs,xkahn/gvfs-cmis,cpatulea/gvfs,philipl/gvfs,gicmo/gvfs,cpatulea/gvfs,xkahn/gvfs-cmis,philipl/gvfs,philipl/gvfs,thiblahute/gvfs,xkahn/gvfs-cmis,halfline/gvfs,halfline/gvfs