When installed, the name is `lightning-hsmtool`. We actually copy
`tools/hsmtool` to `tools/lightning-hsmtool` but that's a silly step
which we should get rid of.
So:
1. Make sure our documentation always refers to it as lightning-hsmtool.
2. Make sure our tests invoke it as `lightning-hsmtool`.
3. Rename the C file.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
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