disallow open(2) in capability mode

Previously open(2) was allowed in capability mode, with a comment that
suggested this was likely the case to facilitate debugging. The system
call would still fail later on, but it's better to disallow the syscall
altogether.

We now have the kern.trap_enotcap sysctl or PROC_TRAPCAP_CTL proccontrol
to aid in debugging.

In any case libc has translated open() to the openat syscall since
r277032.

Reviewed by:	kib, rwatson
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D10850

sys/compat/freebsd32/capabilities.conf

@@ -188,7 +188,6 @@ renameat
 symlinkat
 unlinkat
 freebsd32_utimensat
-open
 pdfork
 pdgetpid
 pdkill

sys/kern/capabilities.conf

@@ -459,14 +459,6 @@ symlinkat
 unlinkat
 utimensat
 
-##
-## Allow entry into open(2). This system call will fail, since access to the
-## global file namespace has been disallowed, but allowing entry into the
-## syscall means that an audit trail will be generated (which is also very
-## useful for debugging).
-##
-open
-
 ##
 ## Process descriptor-related system calls are allowed.
 ##