Fix man proxy in FreeBSD
Fix man proxy in FreeBSD

In FreeBSD, `man` command only searches for man files in MANPATH [1].

[1]: https://www.freebsd.org/cgi/man.cgi?query=man&apropos=0&sektion=0&manpath=FreeBSD+12.0-RELEASE+and+Ports&arch=default&format=html

nodakai/rustup.rs,inejge/rustup.rs,nodakai/rustup.rs,rust-lang/rustup,rust-lang/rustup,inejge/rustup.rs,inejge/rustup.rs,nodakai/rustup.rs,nodakai/rustup.rs,inejge/rustup.rs,inejge/rustup.rs,rust-lang/rustup,rust-lang/rustup,nodakai/rustup.rs,rust-lang/rustup