Supporting relative path

[fyc1007261]

(I have separated the file system modification. I'm terribly sorry that I might not be able to merge all commits into one. I have some other modifications to the kernel that I have commited to my own fork. And the new branch I created today is based on my current master, which already has a lot of old commits, it might be too late for me to merge them into one. Can you select the "squash and merge" option if you would like to accept the PR. That may merge all my all commits into one. Thanks!)

• Support relative path in file system (might contain some concurrency bugs as locks for file systems in tasks are not implemented).
• Add syscall chdir.
• Modify some system calls such as getcwd, open and access. Let them check whether the user is entering a relative path or absolute path then do the corresponding operations.

[lastweek]

Hi @fyc1007261, it's alright! I'll review the code and merge them. However, I will merge some of your commits into one. Again, thank you for your contribution.

[fyc1007261]

Hi @fyc1007261, it's alright! I'll review the code and merge them. However, I will merge some of your commits into one. Again, thank you for your contribution.

It's the first time that I try to contribute to a large open source project. I'm really sorry that it may cause you some trouble.

[lastweek]

Hi @fyc1007261, it's alright! I'll review the code and merge them. However, I will merge some of your commits into one. Again, thank you for your contribution.

It's the first time that I try to contribute to a large open source project. I'm really sorry that it may cause you some trouble.

Hey @fyc1007261 , no worries. Everyone starts from there. It's good you made your first attempt! Besides, I'm just a little (some say extremely :p) picky about code qualify, even for indentation.

Anyway, even if you are doing your own hacking of a kernel, it's suggested to maintain a clean commit history so you know what has been changed by checking commit messages. You can do rebase regularly on your own dev branch (sometimes I got lost from my own random commits).

[fyc1007261]

I found that my current implementation contains some bugs. I'll try to fix it first.