Update po4a to 0.51
- XML: Add an option to escape all quotes, for android. Thanks to Josh Dubois for the patch (Alioth's #315353).
- po4a: Checking po file is fine. Failing if the file does not exist isn't. It makes apt to FTBFS! (Debian's #858934)
- other: Third attempt at installing the manpages to the right location.
Depends on T169
Signed-off-by: Matthias Eliasson <email@example.com>