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

apidoc/ostree-sections.txt

@@ -548,6 +548,7 @@ OstreeSysroot
 ostree_sysroot_new
 ostree_sysroot_new_default
 ostree_sysroot_initialize
+ostree_sysroot_initialize_with_mount_namespace
 ostree_sysroot_get_path
 ostree_sysroot_load
 ostree_sysroot_load_if_changed