nmsgtool -w leaves temporary file behind upon failed start

[rafaelvanoni]

This PR proposes a fix to keep nmsgtool(1) from leaving temporary files behind in case of a failed start. For example:

$ ./src/nmsgtool -w foo -C ch25
bind: Cannot assign requested address
$ file foo
foo: empty

The proposed fix moves the arg parsing of -w foo to after channel validation.

[shw700]

We can omit this test... the existing test scripts are much broader and don't test minor features/fixes/regressions

[shw700]

comment should probably be changed to something like "nmsg inputs and outputs that create files"

[cmikk]

This is a dangling reference to an omitted test: Makefile.am changes need to be backed out.

[cmikk]

The commentary in this diff chunk isn't quite correct. Of the process_args_loop invocations, only add_zsock_input and add_sock_output can create files (unix sockets, to be precise). The rest will only check for existing files or make network connections.

The comment should read /* nmsg inputs and outputs that do not create files */ and the zsock processing moved to the section after validation.

[cmikk]

Comment references inputs, when various inputs, outputs, and modules are validated at this point. It should also be explicit that these file-creating outputs are deferred to avoid leaving stray files if another setup phase exits.

It's also worth noting that even this is only a partial solution: if you have multiple output files and one cannot be opened, the earlier output files will still be left behind. I don't think this bug can be fixed without saving the opened filenames (possibly in the nmsgtool context, maybe globally within src/io.c) and explicitly cleaning up before exit on startup failure.

That explicit cleanup would obviate most of this rearrangement.