mount: add option to specify uid= and gid=

mount: add option to specify uid= and gid=

The kernel needs two numbers, but for the user it's most convenient to provide the
user name and have that resolved to uid and gid.

Right now the primary group of the specified user is always used. That's the most
common case anyway. In the future we can extend the --owner option to allow a group
after a colon.

[I added this before realizing that this will not be enough to be used for user
runtime directory. But this seems useful on its own, so I'm keeping this commit.]