Resize unamebuf[] to avoid warnings about snprintf() not having

Resize unamebuf[] to avoid warnings about snprintf() not having
enough buffer space.  Also change named_os_uname() prototype so
that it is now returning (const char *) rather than (char *).  If
uname() is not supported on a UNIX build prepopulate unamebuf[]
with "unknown architecture".

(cherry picked from commit 4bc3de070fc9c0bc4d8d15d207e0b1352cd20434)