By returning an initialized sigset (instead of taking the set as an output
parameter), these functions can be used to directly initialize the `mask`
parameter of a `Sigaction` instance.
When linking a libc, Zig should defer to the C library for sigset
operations. The pre-filled constants signal sets (empty_sigset,
filled_sigset) are not compatible with C library initialization, so remove
them and use the runtime `sigemptyset` and `sigfillset` methods to
initialize any sigset.
* `doc/langref` formatting
* upgrade `.{ .path = "..." }` to `b.path("...")`
* avoid using arguments named `self`
* make `Build.Step.Id` usage more consistent
* add `Build.pathResolve`
* use `pathJoin` and `pathResolve` everywhere
* make sure `Build.LazyPath.getPath2` returns an absolute path
