* Implement `-U` for Haiku.
This patch also fixes a compilation error on Haiku.
Compiled with `cmake -DBUILD_X11=FALSE ..`
Tested on Haiku-r1beta5.
Takes part of #2072.
* Revert `common.cc`.
A cross-platform solution is already implemented in #2080.
This PR will be rebased *after* the OpenBSD fix was merged.
* Haiku: Remove `wordexp.h`.
Use the `to_real_path()` used for OpenBSD.
* Haiku: Remove `HAIKU_HOME_DIR`.
It was not used.