src/libexec/rc/rc.d/linux, branch release/12.3.0

In 13-CURRENT, the linux rc script uses "nocover" mount option to avoid

2020-08-31T12:14:20+00:00

mounting required filesystems twice.  However, this option hasn't been MFC'd.
Implement its functionality in a different way, by stating the mount point.

Submitted by:	sigsys@gmail.com
Differential Revision:	https://reviews.freebsd.org/D26217

Drop the "nocover" option from the linux rc script; the option

2020-08-29T12:54:17+00:00

is not yet supported in 12-STABLE.

This is a direct commit intended as a temporary workaround.

Reported by:	scf@
Sponsored by:	The FreeBSD Foundation

MFC r362935:

2020-08-27T19:42:52+00:00

Make the linux rc script use linrdlnk by default.

This fixes Linux gettyname(3), with caveats (see PR).

PR:		kern/240767
Sponsored by:	The FreeBSD Foundation

MFC r354690:

2020-08-27T19:41:31+00:00

Add 'linux_mounts_enable' rc.conf(5) variable, to make it possible
to disable mounting Linux-specific filesystems under /compat/linux
when 'linux_enable' is set to YES.

Relnotes:	yes
Sponsored by:	The FreeBSD Foundation

MFC r354458:

2020-08-27T19:40:33+00:00

Extend the linux rc script to mount the neccessary file systems,
set ELF fallback brand, and load pty(4).

Sponsored by:	The FreeBSD Foundation

MFC r352999:

2020-08-27T19:37:28+00:00

Rename etc/rc.d/abi to etc/rc.d/linux; after moving out the SysV IPC stuff
it's entirely linux-specific.

MFC r353054:

Add rcvar back to the linux rc script.  Without it it was enabled
unconditionally.

Sponsored by:	The FreeBSD Foundation