Adopt GNU nano version 2.2.6.

TODO: fix build with UTF-8 support.

configure: WARNING:
*** Insufficient UTF-8 support was detected in your curses and/or C
*** libraries.  If you want UTF-8 support, please verify that your slang
*** was built with UTF-8 support or your curses was built with wide
*** character support, and that your C library was built with wide
*** character support.

Signed-off-by: Rudá Moura <ruda.moura@gmail.com>