Reject empty path arguments - #771

Path arguments become real filenames by concatenation onto "/oldroot/"
or "/newroot/", so an empty path turns into the root directory.

`--bind-try "" DEST` therefore gave the sandbox read/write access to the
whole filesystem, and an empty destination replaced the sandbox root in
the same way. The variants without `-try` failed earlier, because
realpath() rejects an empty source, but the other options that take a
path had no check at all.

Check every path argument during option parsing, and report an empty one
as a usage error.

Signed-off-by: abhinavmir <atg271@gmail.com>