Invalid property name detection locale issue

Somewhat related to bug 1807:

libsvn_client/prop_commands.c:is_valid_prop_name() tries to detect non-ASCII
names by using isalpha() on a UTF-8 string, but isalpha() is locale-specific.

This allows another way to create properties which mod_dav_svn will reject as
the PROPPATCH request includes invalid XML element names:

e.g.

$ LC_CTYPE=en_US svn propset foo`/bin/echo -e '\265'`bar fish somefile