osproc.execCmdEx now takes an optional input for stdin, env, workingDir

[timotheecour]

osproc.execCmdEx now takes an optional input for stdin. This is a common use case, and avoids the bashism input | cmd for common cases where input is small enough to avoid blocking (eg on OSX, 140_000 does not block but 150_000 blocks)

• as noted, writing stdin by chunks (in case of large stdin) would require selectors (kqueue/epoll etc) otherwise you'll end up blocking either when writing to stdin (pipe full) or reading from stdout (output empty). This can be fixed in future work (actually I'm advocating for a osproc2 that fixes all the design issues with osproc, but that's left as future work).

• the runnableExample comment is intentionally not a doc comment but may be useful for people looking at the code (it hits ^[[0m is inserted in stderr for echo code | nim c - timotheecour/Nim#152 depending on your user config)

• the when (NimMajor, NimMinor, NimPatch) < (1, 3, 3): doAssert input.len == 0 is a compromise for the sake of simplicity (avoiding to rewrite a separate template etc)

• env, workingDir are also added for more consistency with rest of osproc and to avoid more bashisms (eg "cd $dir && cmd" or other existing kludges)

[Araq]

This needs to have a test so that we know it works on all CI'ed OSes.

[timotheecour]

PTAL, done (ya, nim doc isn't run on all CI OS's anymore...)