improved: move out code of fetching and installing to function

To improve later (add command `fetch`)
fixed: bash error when no file received
improved: option "--fetch" added

To fetch files only (do not install).
code cleanup: removed trailing whitespaces
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.
improved: print error messages to stderr
improved: exit if cache dir is not avalable
refactoring: use INSTALLED_DB var
improved: use var SETUP_DIR
refactoring: explicitly use "-v" option for awk
improved: added --verbose option

Use "--verbose" option to show versions for "show" and "find" commands.
improved: now package name can contain spaces

This will be used later (for `outdated` command).
improved: added "--wget-options" option
improved: added "outdated" command

To show outdated packages
improved: added "upgrade" command

To upgrade outdated packages
improved: added "--prefix" option to be able to debug script

And run on Linux, too.
`cygpath` is emulated with a function.