Some keymaps (e.g.  polish) use the AltGr key as a modifier to enter
diacritised letters.

For example:
AltGr + c produces ć
AltGr + e produces ę

Thus, to be able to enter capital letters, the keyboard driver must
recognize combinations like:

Shift + AltGr +c which gives Ć
Shift + AltGr + e which gives Ę

This patch adds another kbtab to /sys/src/9/pc/kbd.c (kbtabshiftaltgr)
and adds support for Shift + AltGr combinations.

The /sys/lib/kbmap/pl is a polish keymap that utilizes the new kbtab.