posix: remove some iso-8859-encoded characters

With the increasing adoption of UTF-8, modern editors may (will?)
replace iso-8859-encoded characters in the range 0x80..0xff with
their UTF-8 equivalent, as will mailers and other tools.  This breaks
our testsuite and corrupts patches.

So, this patch starts replacing these problematic characters with
\OCTal sequences instead (adding support for those in tst-fnmatch.c)
or with plain ASCII characters (PTESTS).

Reviewed-by: Carlos O'Donell <carlos@redhat.com>