* shell: expand `~` in local file path arguments
The weed shell parses commands itself instead of going through an OS
shell, so a path like `~/Downloads/foo.meta` was passed verbatim to
`os.Open`, which fails because no `~` directory exists. Users had to
spell out absolute home paths in every command.
Add an `expandHomeDir` helper that resolves a leading `~` or `~/...` to
the user's home directory, and run user-supplied local file paths in
the affected shell commands through it:
fs.meta.load (positional file)
fs.meta.save (-o)
fs.meta.changeVolumeId (-mapping)
s3.iam.export (-file)
s3.iam.import (-file)
s3.policy (-file)
s3tables.bucket (-file)
s3tables.table (-file, -metadata)
volume.fsck (-tempPath)
Filer-namespace path flags (`-dir`, `-path`, `-locationPrefix`, etc.)
are unaffected; they live in the filer, not on the local FS.
* shell: reuse util.ResolvePath instead of a new helper
util.ResolvePath already does tilde expansion; drop the local
expandHomeDir helper and route every shell call site through it.
