Add logic for parsing and matching against our planned format for
AcceptEnv values. Namely, this supports direct matches against string
values and matching where * and ? are treated as wildcard characters
which match against an arbitrary number of characters and a single
character respectively.
Actually using this logic in non-test code will come in subsequent
changes.
Updates https://github.com/tailscale/corp/issues/22775
Signed-off-by: Mario Minardi <mario@tailscale.com>
