don't write openpty results until success is determined

not only is this semantically more correct; it also reduces code size
slightly by eliminating the need for the compiler to assume the
possibility of aliasing.
block pthread cancellation in openpty function

being a nonstandard function, this isn't strictly necessary, but it's
inexpensive and avoids unpleasant surprises. eventually I would like
all functions in libc to be safe against cancellation, either ignoring
it or acting on it cleanly.
overhaul forkpty function using new login_tty

based on discussion with and patches by Felix Janda. these changes
started as an effort to factor forkpty in terms of login_tty, which
returns an error and skips fd reassignment and closing if setting the
controlling terminal failed. the previous forkpty code was unable to
handle errors in the child, and did not attempt to; it just silently
ignored them. but this would have been unacceptable when switching to
using login_tty, since the child would start with the wrong stdin,
stdout, and stderr and thereby clobber the parent's files.

the new code uses the same technique as the posix_spawn implementation
to convey any possible error in the child to the parent so that the
parent can report failure to the caller. it is also safe against
thread cancellation and against signal delivery in the child prior to
the determination of success.
fix signedness of UINT32_MAX and UINT64_MAX at the preprocessor level

per the rules for hexadecimal integer constants, the previous
definitions were correctly treated as having unsigned type except
possibly when used in preprocessor conditionals, where all artithmetic
takes place as intmax_t or uintmax_t. the explicit 'u' suffix ensures
that they are treated as unsigned in all contexts.
move wint_t definition to the shared part of alltypes.h.in