Fix "not a tty" bug on Solaris 10

Fix "not a tty" bug on Solaris 10

* configure.ac (PTY_OPEN): Define to plain 'open'
on SVR4-derived hosts, so that the O_CLOEXEC flag isn't set.
* src/process.c (allocate_pty): Set the O_CLOEXEC flag after
calling PTY_TTY_NAME_SPRINTF, for the benefit of SVR4-derived
hosts that call grantpt which does its work via a setuid subcommand
(Bug#19191, Bug#19927, Bug#20555, Bug#20686).
Also, set O_CLOEXEC even if PTY_OPEN is not defined, since it
seems relevant in that case too.