This adds 2 tests. The first checks that ssh config data is cached and
does not update if the file changes afterward. The second checks that
the `ReloadConfigs()` function will discard the cached data and load the
current config file contents.
See the description in the CHANGELOG - we were handling this
incorrectly and attaching whitespace to the end of values where that
didn't make much sense to do.
IdentityFile among others supports being provided multiple times and
aggregated across, potentially, multiple files. Support that workflow
by adding GetAll and GetAllStrict alongside the current functions.
The error handling is nonexistent and there's no easy way to get
data out. But we can parse a SSH config file into a Go struct, and
roundtrip that struct back to a file that looks (roughly) the same.