improved: replace `cd`s with `pushd`/`popd`
improved: added option "-F" (short alias for "--force")
refactoring: introduced variables for better "improvability"
improved: "desc" file is in /tmp now
fixed: some dependent packaged are stored in into its own directory (but not subdirectory)

For example w32api-{headers,runtime} packages have paths
    release/w32api/w32api-headers/<filename>
    release/w32api/w32api-runtime/<filename>
but when they are retrieved they are stored into
    release/w32api-headers/<filename>
    release/w32api-runtime/<filename>
tree

Though Cygwin installer saves into original tree
    release/w32api/w32api-headers/<filename>
    release/w32api/w32api-runtime/<filename>

Make `apt-cyg` behave as Cygwin installer does.