os: implement replacement for Waitmsg

[rsc]

Waitmsg is hopelessly unportable.
Define a new struct or interface.

Replace wait arguments (WUNTRACED ...) too.

[bradfitz]

Comment 1:

Dup of issue #2823.
Will close that one.

[bradfitz]

Comment 2:

Issue #2823 has been merged into this issue.

[robpike]

Comment 3:

Text by the primary fitz from closed 2383:
Problems with the bits in *os.Process Wait(options int)
1) they're non-portable
2) they're a mix of syscall bit numberspace and Go-assigned numbers (1 << 20 for
os.WRUSAGE)
We should clean this up somehow:
a) remove the options parameter to Wait
b) keep the options parameter, but document the bits as all Go-defined portable bits
(like FileMode)
c) document that they're all syscall bits and pass them along, but don't provide alias.
force callers to use syscall.WNOHANG, etc.  that leaves WRUSAGE, but we could just
always return that, and remove WRUSAGE.

[dsymonds]

Comment 4:

Owner changed to @robpike.

Status changed to Started.

[robpike]

Comment 5:

Fixed by http://golang.org/cl/5673077/.

Status changed to Fixed.