MFC r303464 (by brooks@):

Don't create pointless backups of generated files in "make sysent".

Any sensible workflow will include a revision control system from which
to restore the old files if required. In normal usage, developers just
have to clean up the mess.

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Continue to introduce Capsicum Capability Mode support:

Add a new system call flag, SYF_CAPENABLED, which indicates that a
particular system call is available in capability mode.

Add a new configuration file, kern/capabilities.conf (similar files
may be introduced for other ABIs in the future), which enumerates
system calls that are available in capability mode. When a new
system call is added to syscalls.master, it will also need to be
added here (if needed). Teach sysent parts to use this file to set
values for SYF_CAPENABLED for the native ABI.

Reviewed by: anderson
Discussed with: benl, kris, pjd
Obtained from: Capsicum Project
MFC after: 3 months

Adjust the all target message (but maybe all: sysent is better?

Turns out this file was how we make sysent stuff, so add that part only back...