syscalls: syscall id lookup and dispatch table improvements

syscalls: syscall id lookup and dispatch table improvements

Use the syscall id from event creation as key to index the filter
bitmap rather than lookup the syscall id using the syscall name as
key. This additional lookup is completely redundant.

For unknown syscalls, add the unknown syscall event to the unknown
system call dispatch table only when it matches an enabler: the only way
it matches an enabler is because a wildcard is used to enabled all
system calls (until we eventually support enabling system calls by
number).

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I61ce9d447e5056976605ab0e048ff5f9cd8ff484