syslogd: Move prop_filter allocation into function

[jfree]

Summary

Clean up the cfline() function by moving the prop_filter memory
allocation into the prop_filter_compile() function.

[markj]

usr.sbin/syslogd/syslogd.c

2851–2852

These frees are in the wrong order.

It'd be nicer to have a dedicated error path:

goto error;
...
error:
    free(pfilter->pflt_re); // pfilter is zero-initialized, and free(NULL) is fine.
    free(pfilter);
    return (NULL);

2930–2931

Why not move the strdup() into prop_filter_compile() as well?

[markj]

usr.sbin/syslogd/syslogd.c

2930–2931

If you adopt this suggestion, D41378 is no longer needed.

[jfree]

Address Mark's comments

[jfree]

In prop_filter_compile(), the filter string pointer is modified so free()'ing it leads to unintended behavior. Save a filter_begpos pointer and free() that during cleanup.