ptsname_r: Work around bug on Android 4.3.

ptsname_r: Work around bug on Android 4.3.

* m4/ptsname_r.m4 (gl_FUNC_PTSNAME_R): Define
HAVE_ESSENTIALLY_WORKING_PTSNAME_R. Test whether the return value is
correct.
* lib/ptsname_r.c (__ptsname_r): If HAVE_ESSENTIALLY_WORKING_PTSNAME_R
is defined, just fix the return value.
* doc/glibc-functions/ptsname_r.texi: Mention the Android bug. Reword:
The behaviour of musl libc is nothing to be "fixed", since it is
compliant with the next POSIX standard.