MFC r331616: vfs_donmount: in certain cases try r/o mount if r/w mount fails

If the operation is not an update, if neither r/w nor r/o mode is
explicitly requested, if the error code hints at the possibility of the
media being read-only, and if the fallback is allowed, then we can try
to automatically downgrade to the readonly mode.

This is especially useful for auto-mounting of removable media that
sometimes can happen to be write-protected.

The fallback to r/o is not enabled by default. It can be requested on a
per-mount basis with a new mount option, 'autoro'. Or it can be
globally allowed by setting vfs.default_autoro.

stable/10 note: this branch does not have SYSCTL_BOOL, so SYSCTL_INT is
used instead.

Relnotes: yes

MFC r315775:

Add a post-humous manpage for cd9660(5), the ISO-9660 file system

Describe (briefly) how to compile the filesystem into the kernel and
load as a module.

Reference cd9660(5) in mount(8) and mount_cd9660(8).

MFC r289229:

Replace references to /dev/acd0 with /dev/cd0

atapicd(4) was replaced by cd(4) with the atacam work done by
mav@ and then removed in r249083

X-MFC to: stable/10

MFC r275680:

Add fstyp(8). This utility, named after its SVR4 counterpart, detects
filesystems. It differs from file(1) in that it gives machine-parseable
output, it outputs filesystem labels, doesn't get confused by other
formats metadata, and runs in Capsicum sandbox.

Differential Revision: https://reviews.freebsd.org/D1255
Relnotes: yes
Sponsored by: The FreeBSD Foundation

MFC r274857:

Cross-reference automount(8) from mount(8).

Sponsored by: The FreeBSD Foundation

MFC r269627:

Mention tmpfs(5)

PR: 192389
Submitted by: yaneurabeya@gmail.com
Approved by: hrs (mentor)

MFC r270209:

Add description for the "automounted" mount flag.

Reviewed by: emaste@
Sponsored by: The FreeBSD Foundation

MFC r275680:

Add fstyp(8). This utility, named after its SVR4 counterpart, detects
filesystems. It differs from file(1) in that it gives machine-parseable
output, it outputs filesystem labels, doesn't get confused by other
formats metadata, and runs in Capsicum sandbox.

Differential Revision: https://reviews.freebsd.org/D1255
Relnotes: yes
Sponsored by: The FreeBSD Foundation

MFC r274857:

Cross-reference automount(8) from mount(8).

Sponsored by: The FreeBSD Foundation

MFC r269627:

Mention tmpfs(5)

PR: 192389
Submitted by: yaneurabeya@gmail.com
Approved by: hrs (mentor)

MFC r270209:

Add description for the "automounted" mount flag.

Reviewed by: emaste@
Sponsored by: The FreeBSD Foundation