linux: add sysctl compat.linux.use_emul_path

This is a step towards facilitating jails with only Linux binaries.
Supporting emul_path adds path lookups which are completely spurious
if the binary at

linux: add sysctl compat.linux.use_emul_path

This is a step towards facilitating jails with only Linux binaries.
Supporting emul_path adds path lookups which are completely spurious
if the binary at hand runs in a Linux-based root directory.

It defaults to on (== current behavior).

make -C /root/linux-5.3-rc8 -s -j 1 bzImage:

use_emul_path=1: 101.65s user 68.68s system 100% cpu 2:49.62 total
use_emul_path=0: 101.41s user 64.32s system 100% cpu 2:45.02 total

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linux: consistently use LFREEPATH instead of open-coding it

Add linux_madvise(2) instead of having Linux apps call the native
FreeBSD madvise(2) directly. While some of the flag values match,
most don't.

PR: kern/230160
Reported by: markj
Reviewed by: mark

Add linux_madvise(2) instead of having Linux apps call the native
FreeBSD madvise(2) directly. While some of the flag values match,
most don't.

PR: kern/230160
Reported by: markj
Reviewed by: markj
Discussed with: brooks, kib
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D25272

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Introduce arm64 linuxulator stubs

This provides stub implementations of arm64 Linux vdso and machdep,
ptrace, and futex sufficient for executing an arm64 Linux 'hello world'
binary.

Reviewed by: an

Introduce arm64 linuxulator stubs

This provides stub implementations of arm64 Linux vdso and machdep,
ptrace, and futex sufficient for executing an arm64 Linux 'hello world'
binary.

Reviewed by: andrew
Sponsored by: Turing Robotic Industries
Differential Revision: https://reviews.freebsd.org/D15832

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