Added test and comments for poll_syscall

[qianxichen233]

Description

This PR added several tests and comments for poll_syscall. The test being added is very simliar to select_syscall, as both syscalls serve a very similar purpose.

Fixes # (issue)

Several minor issues in poll_syscall are fixed in this PR:

1. POLLPRI flag should be used for exception fds instead of POLLERR
2. revents of all fds are cleared at the beginning of the syscall, in order to follow the standard
3. negative fds should be ignored
4. invalid fd should not raise the error directly, instead, it should just ignore it and set the corresponding flag in revent

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

• Test A - ut_lind_net_poll_bad_input
• Test B - ut_lind_net_poll_timeout
• Test C - ut_lind_net_poll (this test is marked as ignore since an unfixed bug will panick 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)

[davidge20]

Great tests! Left some minor comments for more context and typos

[namanlalitnyu]

Looks good to me, just two minor comments.
Very thorough tests!

[davidge20]

LGTM!

[yashaswi2000]

LGTM, really great job!

[rennergade]

Minor comment but this is good to go. Great job!