fakedate: simplify logic

fakedate: simplify logic

Using -ef to check for the same file is nicer than relying on a magic
symlink-to-fakedate.

Notice that -ef isn't stricly posix (but supported by bash/dash/zsh), so
I've changed the shebang to /bin/bash.

While we are at it, restructure the logic to do a single exec at the end
instead of handling the epoch/!epoch cases differently for simplicity.

With that out of the way we can directly install it as $HOST/usr/bin/date
instead of the fakedate / date symlink.

[Peter: drop IFS=: change]
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>