Bump buildinfo.Version to 0.16.3

Backport x/sys upgrade to 0.16 release

Backport just x/sys upgrade from c99d6e1 to v0.16-release branch

Alternative to & closes elves#1388

Set VersionSuffix empty.

This was not done correctly for 0.16.0, so also make this 0.16.1.

Fix mods/path test on Go 1.17.

The TempDir and TempFile functions now return filenames starting with "./" when
the directory is ".".

Merge branch 'master' into v0.16-release

Stick to \ as path divider in doc for Windows paths.

Add doc for the Elvish command.

The new page documents the new XDG-compliant search paths, among other things.

This resolves elves#383.

Fix highlighting of command names with explicit builtin:.

Fix highlighting of command names with explicit builtin:.

Fix the panic when referencing a module that calls edit:add-var.

This fixes elves#1225.