If `slave` is negative, neither `dup2(slave,0)` or `close(slave)` should
be executed. I believe this check is completely useless.

getlogin is only called if USER environment variable is not set,
but if getlogin returns NULL in that case, then do not call
getpwnam, and assume /bin/sh as shell.

Mentioned in comment to bug 20586.

If `posix_openpt` is available, also `ptsname` should be available.

Although glibc `ptsname_r` man page mentions Tru64 and HP-UX, this
function appears to be declared obsolete on both.

- start a new session
- obtain the new controlling terminal
- drop privileges
- finally, `exec`

This reverts commit 054a412d540e7ed2de63d68da753f585ea6616c3.
SIGCHLD `waidpid`, `waitpid_lock` and related code, have been removed
at ruby/ruby#7527.

`:nodoc:` directive does not work at method definition in C, and must
be at the implementation function.  That is, there is no way to make
one method visible and another method sharing the implementation
invisible at the same time.