Improve docs for linux-syscall and fix "uname" command

[yunwei37]

linux-syscall passes #![deny(missing_docs)].

reference:

• system reference manuals section 2: System calls

fix #26 : The "uname" command shows up incorrect results

[coveralls]

Pull Request Test Coverage Report for Build 197145099

• 0 of 4 (0.0%) changed or added relevant lines in 2 files are covered.
• 32 unchanged lines in 8 files lost coverage.
• Overall coverage increased (+0.02%) to 29.726%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
linux-syscall/src/task.rs	0	1	0.0%
linux-syscall/src/misc.rs	0	3	0.0%

Files with Coverage Reduction	New Missed Lines	%
linux-syscall/src/file/stat.rs	1	0%
linux-syscall/src/time.rs	1	0%
linux-syscall/src/task.rs	2	9.56%
linux-syscall/src/file/dir.rs	4	0%
linux-syscall/src/lib.rs	5	28.68%
linux-syscall/src/misc.rs	5	14.29%
../../../../../usr/share/rust/.cargo/registry/src/git.luolix.top-1ecc6299db9ec823/bitflags-1.2.1/src/lib.rs	6	36.27%
linux-syscall/src/file/file.rs	8	13.45%

Totals
Change from base Build 193518124:	0.02%
Covered Lines:	3714
Relevant Lines:	12494

---

💛 - Coveralls

[wangrunji0408]

note: this feature is not correctly implemented.

[coveralls]

Pull Request Test Coverage Report for Build 195623298

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

• 5 of 29 (17.24%) changed or added relevant lines in 5 files are covered.
• 162 unchanged lines in 9 files lost coverage.
• Overall coverage increased (+0.08%) to 29.781%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
linux-syscall/src/vm.rs	4	5	80.0%
linux-syscall/src/time.rs	0	3	0.0%
linux-syscall/src/misc.rs	1	7	14.29%
linux-syscall/src/lib.rs	0	7	0.0%
linux-syscall/src/task.rs	0	7	0.0%

Files with Coverage Reduction	New Missed Lines	%
linux-syscall/src/file/stat.rs	1	0.0%
../../../../../usr/share/rust/.cargo/registry/src/git.luolix.top-1ecc6299db9ec823/bitflags-1.2.1/src/lib.rs	3	36.27%
linux-syscall/src/file/dir.rs	4	0.0%
linux-syscall/src/time.rs	4	0.0%
linux-syscall/src/file/file.rs	8	13.45%
linux-syscall/src/vm.rs	14	67.31%
linux-syscall/src/misc.rs	16	14.58%
linux-syscall/src/lib.rs	54	29.55%
linux-syscall/src/task.rs	58	9.42%

Totals
Change from base Build 193518124:	0.08%
Covered Lines:	3720
Relevant Lines:	12491

---

💛 - Coveralls