Improve doc and add some simple trivial test in linux-loader

[yunwei37]

• linux-loader passes #![deny(missing_docs)]
• add some simple test using busybox command to improve the coverage in linux-loader:
  • fs
    • copy and remove
    • link and unlink
    • create and remove dir
    • readfile
  • date
  • mkdir

It seems most command in the busybox need more syscalls.

[coveralls]

Pull Request Test Coverage Report for Build 197867979

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 22 of 24 (91.67%) changed or added relevant lines in 2 files are covered.
• 4 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+2.9%) to 32.641%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
linux-loader/src/lib.rs	0	1	0.0%
linux-loader/src/main.rs	22	23	95.65%

Files with Coverage Reduction	New Missed Lines	%
../../../../../usr/share/rust/.cargo/registry/src/github.com-1ecc6299db9ec823/async-std-1.6.2/src/sync/mutex.rs	4	77.78%

Totals
Change from base Build 197305326:	2.9%
Covered Lines:	4085
Relevant Lines:	12515

---

💛 - Coveralls