Issue #2: Add hostname wrapper binaries

[HaitaiNg]

Description:
Issue : #2

This pull request adds wrapper binaries for domainname, dnsdomainname, ypdomainname, nisdomainname each in bin that call into the hostname library (hostname -d or hostname -y). The domainname, nisdomainname, and ypdomainname are aliases for the NIS/YP domain aname. I implemented them as thin wrapper binaries over the existing hostname implementation to avoid duplicated logic and reduce long term drift.

Testing

cargo test --test wrapper_binaries

Additional Validation

Build binaries:
cargo build --bins 

Run a build binary directly 
target/debug/hostname -d 
target/debug/dnsdomainname --help
target/debug/nisdomainname --help
target/debug/ypdomainname --help
target/debug/domainname --help 

You could also use cargo:
cargo run --bin domainname -- --help

[HaitaiNg]

This is my first pull request for a rust project. Any feedback would be appreciated. Thanks in advance.