jtag_vpi: multiple improvements

jtag_vpi: multiple improvements

- Fix: Proper handling of read_socket() and write_socket()
in case of "partial" read/write.

- Added low-level JTAG IO debug capability (_DEBUG_JTAG_IO_)

- Zero-fill packet buffers, avoid sending pieces of uninitialized
memory over the network (memset struct vpi_cmd)

- Use close_socket() instead of close() - needed for Win32

- Fixed usage messages of jtag_vpi_command_handlers

Change-Id: I8bd19bc5c9512fe8e798600212e8a95213f50f5b
Signed-off-by: Jan Matyas <matyas@codasip.com>
Reviewed-on: http://openocd.zylin.com/5177
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>