added tests and comments for epoll syscalls

[qianxichen233]

Description

This PR added some tests and comments for epoll related syscalls (epoll_create_syscall, epoll_ctl_syscall and epoll_wait_syscall).

Fixes # (issue)

Several minor issues in epoll are fixed in this PR:

1. fixed some wrongly returned errno, and added a few errnos that can be easily supported
2. added check for fd range to prevent panic from get_filedescriptor
3. fixed when calling epoll_ctl on EPOLL_CTL_DEL flag with non-existing fd would causing panic
4. fixed the error case that EINVAL should be returned when maxevents is smaller or equal 0 (previously is smaller 0).
5. fixed misuse of EPOLLERR, should be EPOLLPRI instead
6. fixed maxevents issue in epoll_wait. Previous behavior of maxevents is that it only checks the first maxevents fds for readability/writability. E.g. if there are 10 file descriptors, and 5 of them are ready. Passing maxevents of 5 would cause epoll only monitors the first 5 file descriptor, and report for its readability/writability. Therefore, the number of ready fd could be less than 5. However, the correct behavior should be that epoll should always report first 5 ready file descriptor, in this case, its return value should always be 5.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

All the fixed behavior passed the test. (before the fix, it won't pass the test)

• Test A - ut_lind_net_epoll_create_bad_input
• Test B - ut_lind_net_epoll_ctl_bad_input
• Test C - ut_lind_net_epoll_wait_bad_input
• Test D - ut_lind_net_epoll_maxevents_arg
• Test E - ut_lind_net_epoll_timeout
• Test F - ut_lind_net_epoll (this test is marked as ignore since an unfixed bug will panic the test)

Checklist:

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• Any dependent changes have been added to a pull request and/or merged in other modules (native-client, lind-glibc, lind-project)

[ve1nard]

some changes requested

[namanlalitnyu]

Some minor doubts related to the variables.

[namanlalitnyu]

Just one minor comment, but overall looks good!

[yashaswi2000]

few minor comments, good job with the extensive tests

[qianxichen233]

@yashaswi2000 just resolved these comments, could you check again?

[yashaswi2000]

LGTM

[rennergade]

This is really thorough. Great job! One minor request.