Use makedoc generated texinfo documentation for reentrant syscalls

Use makedoc generated texinfo documentation for reentrant syscalls

We use texinfo nodes beginning with an underscore in several other places, so
revert this ancient workaround for a no longer existing bug, and use the makedoc
generated texinfo for reentrant versions of syscalls, rather than handwritten
documentation.

Also alphabetically sort these functions.

Also add documentation for _execve_r, _getpid_r, _kill_r and _times_r functions,
whose non-reentrant versions are documented as stubs

v2:
Keep _open64_r, _lseek64_r and _fstat64_r functions under texinfo conditional STDIO64
Add _stat64_r function likewise.

Notes:

1. The handwritten prototypes give the reentrancy structure pointer as of type
void *, rather than the presumably more correct struct __reent *

2. The fcntl, gettimeofday, mkdir and rename functions are not documented as
stubs, so I haven't added the reentrant versions either

Signed-off-by: Jon TURNEY <jon.turney@dronecode.org.uk>