Add initial support for SFTP

3rdp/build/GNUmakefile

3rdp/build/cl-channel-select-both.patch

3rdp/build/cl-fix-shell-exec-types.patch

3rdp/build/cl-ssh-eof-half-close.patch

src/sbbs3/GNUmakefile

src/sbbs3/answer.cpp

src/sbbs3/ctrl/sbbsctrl.bpr

src/sbbs3/js_uifc.c

src/sbbs3/main.cpp

src/sbbs3/objects.mk

src/sbbs3/sbbs.h

src/sbbs3/sbbs.vcxproj

src/sbbs3/sbbscon.vcxproj

src/sbbs3/sftp.cpp

src/sbbs3/sftp.h

src/sbbs3/startup.h

src/sftp/sftp.h

src/sftp/sftp.props

src/sftp/sftp_attr.c

src/sftp/sftp_client.c