audit(4): Syscalls dealing with change of a file-mode

[aniketp]

Summary

The following revision introduces system calls from file-attribute-modify audit class
which deal with modifying the mode of a given file. System calls are:

• chmod(2)
• fchmod(2)
• lchmod(2)
• fchmodat(2)

Test Plan

Execute make && make install from test/sys/audit.
Execute kyua test from /usr/tests/sys/audit. All testcases should succeed.