sys: Remove $FreeBSD$: one-line sh pattern

Remove /^\s*#[#!]?\s*\$FreeBSD\$.*$\n/

sysent: regenerate files every time

This forces everything to be regenerated by marking makesyscalls.lua
as .PHONY, which may be helpful in, e.g., rebase scenarios, which may
have unexpected effects on mtimes but still really need a regen.

For now we apply this to the default sysent target, which is ran
manually and not as a part of buildkernel. We can reconsider this if
someone manually running it has issues with the runtime, but the top
level sysent target can run each in parallel; I recommend -j4 or so.

Requested by: kib
Reviewed by: brooks, kib (both earlier version)
Differential Revision: https://reviews.freebsd.org/D33357

syscalls.master: switch to CAPENABLED flags

Switch the main syscall table to use CAPENABLED flags rather than
capabilities.conf. This avoid synchronization issues between
syscalls.master and capabilities.conf (e.g. when renaming a syscall
during development).

For now, move capabilities.conf to sys/compat/freebsd32 and use it
there. Use of sys/compat/freebsd32/syscalls.master should be replaced
by makesyscalls.lua enhancements to allow the main one to be used.

This change results in no changes to generated files after running
`make sysent`.

Reviewed by: kevans, emaste
MFC after: 1 week
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D31350

Rebuild sysent when capabilities.conf is updated.

Reviewed by: brooks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D25571

sysent.mk: split interpreter out of target command

The main objective here is to make it easy to identify what needs to change
in order to use a different sysent generator than the current Lua-based one,
which may be used to MFC some of the changes that have happened so we can
avoid parallel accidents in stable branches, for instance.

As a secondary objective, it's now feasible to override the generator on a
per-Makefile basis if needed, so that one could refactor their Makefile to
use this while pinning generation to the legacy makesyscalls.sh. I don't
anticipate any consistent need for such a thing, but it's low-effort to
achieve.

sysent targets: further cleanup and deduplication

r355473 vastly improved the readability and cleanliness of these Makefiles.
Every single one of them follows the same pattern and duplicates the exact
same logic.

Now that we have GENERATED/SRCS, split SRCS up into the two parameters we'll
use for ${MAKESYSCALLS} rather than assuming a specific ordering of SRCS and
include a common sysent.mk to handle the rest. This makes it less tedious to
make sweeping changes.

Some default values are provided for GENERATED/SYSENT_*; almost all of these
just use a 'syscalls.master' and 'syscalls.conf' in cwd, and they all use
effectively the same filenames with an arbitrary prefix. Most ABIs will be
able to get away with just setting GENERATED_PREFIX and including
^/sys/conf/sysent.mk, while others only need light additions. kern/Makefile
is the notable exception, as it doesn't take a SYSENT_CONF and the generated
files are spread out between ^/sys/kern and ^/sys/sys, but it otherwise fits
the pattern enough to use the common version.

Reviewed by: brooks, imp
Nice!: emaste
Differential Revision: https://reviews.freebsd.org/D23197