add last_disconnect_cause

FileAccess.cc, FileAccess.h, src/Fish.cc, src/Fish.h, src/Http.cc,
Http.h, NetAccess.cc, SFtp.cc, SFtp.h, SSH_Access.cc, SSH_Access.h,
ftpclass.cc, ftpclass.h: add last_disconnect_cause; display it
for 5 seconds in the status.