Build: remove slashes after DESTDIR from commands in uninstall target

Rather than setting the padding.

And adjusting the geometry of items.

Renaming calcmenu() to setupmenu() and shrinking it by calling two
new functions: setupmenusize() and setupmenupos().

I moved all the geometry related code to calcmenu().

The pro is that geom is now a local variable rather than global;
and that all geometry-related code is now in one place.

The con is that it weakens performance a little bit since some
variables, such as the border, have to be reset after the window
creation.

• Parsing of stdin was simplified by using conventional stdlib functions
  rather than parsing the stdin byte-by-byte.
• Splited parsing of textual input and building of internal data
  structures into two different functions.

When the item->label and item->output are the same, XMenu now only
strdup() the item->label and make item->output equal to item->label.
This saves memory when the user do not write a output specification
in the input.

• Updated manual page (removing mention to the removed -w option).
• Removed unused variables (dc.d).
• Freeing item's label and output.

The position didn't take into account the menu borders.
Also, freeing allocated menus and items.

They are local now.

I will remove the -w (windowed) option from the master branch, for it
raises more problems than solutions, I will maintain it in the windowed
branch.

Every time a window is opened, its position is recalculated with
recalcmenu().

Xmenu didn't wait for another process to ungrab the poiter or keyboard,
ending up in erroneous behavior.  Now it waits.

The script now runs in the background.

You must grab the keyboard after reading the input.