sysroot: Add an API to initialize with mountns

sysroot: Add an API to initialize with mountns

This lowers down into the C library some logic we
have in the binary/app logic, in prep for having more Rust-native
CLI code in https://github.com/ostreedev/ostree-rs-ext/pull/412

Basically we want to *ensure* a mount namespace by invoking
`unshare()` if necessary, instead of requiring our callers
to do this dance.

This also helps fix e.g.
Closes: https://github.com/ostreedev/ostree/issues/2769