deps: Disable dependency default-features by default; upgrade dependencies to latest

[bryantbiggs]

• Disable dependency default features with default-features = false, forcing explicit feature opt-in to ensure only the necessary dependencies are pulled in. Dependencies were updated to the latest during this change
  • Closes containerd_client pulls in server side of tonic #451
• Replace 3rd party crates with native std library functionality; fewer dependencies to update and maintain

Summary:

- 263 total dependencies on main → 251 on this branch                                                                                                                                                                                                                                                                   
- Net reduction: 12 dependencies (14 removed, 2 added)
                                                                                                                                                                                                                                                                                                                        
Removed crates:                                                                                                                                                                                                                                                                                                         
- lazy_static → replaced by std::sync::LazyLock
- os_pipe → replaced by std::io::pipe()
- page_size → replaced by direct libc::sysconf call
- prctl → replaced by direct libc::prctl call
- path-absolutize / path-dedot → replaced by std::path::absolute()
- parking_lot / parking_lot_core / lock_api / scopeguard / redox_syscall → transitive deps of parking_lot, no longer needed
- pulldown-cmark / pulldown-cmark-to-cmark / unicase → transitive deps dropped

However, this isn't the complete picture. Per crate, it will vary based on what is removed due to scoping the features used by each crate.

Fixes pre-existing correctness and hygiene issues discovered during the dependency cleanup work.

Let me know if this commit needs to move to a separate PR:

• Add error handling for sysconf(_SC_PAGESIZE). The raw sysconf result was cast directly to usize, silently producing usize::MAX on error and causing incorrect page size checks for overlay mount compaction.
• Fix double-close in PipedIo::close_after_start. close_after_start() closed a raw fd while the owning type still held it, leading to a double-close on drop. If the fd number was reused between the two closes, this could silently close an unrelated fd belonging to another part of the process.
• Fix unused variable in prefix_filter. Dead if-let binding replaced with a simpler starts_with() check.
• Simplify async Pipe to use std::io::pipe(). The async pipe unnecessarily went through tokio::net::unix::pipe and then converted back to blocking fds. Now uses std::io::pipe() directly, consistent with the sync pipe.
• Remove blanket #![allow(unused)] from mount_linux.rs. The file-level suppression hid real warnings. Replaced with targeted cfg gates on feature-specific imports and a scoped allow on MountInfo whose fields are populated during parsing for completeness.
• Also fixed a couple minor unused variable/assignment warnings that surfaced.

[bryantbiggs]

cc @mxpv if you have some time