Hurd: fixes for ptsname and ptsname_r

Hurd: fixes for ptsname and ptsname_r

ptsname_r on failure returns the value that is also set as errno; furthermore,
add more checks to it:
- set errno and return it on __term_get_peername failure
- set errno to ERANGE other than returning it
- change the type of PEERNAME to string_t, and check its length with __strnlen

In ptsname:
- change the type of PEERNAME to string_t
- do not set errno manually, since ptsname_r has set it already