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

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

Fix an error in setfacl(1) that manifested like this:

# setfacl -m u:trasz:rwx x
# setfacl -m u:root:rwx x
# getfacl x
# file: x
# owner: root
# group: wheel
user::rw-
user:root:rwx
user:trasz:rwx
group::r--
mask::rwx
other::r--
# setfacl -m u:root:rwx x
setfacl: x: acl_calc_mask() failed: Invalid argument
setfacl: x: failed to set ACL mask

For NFSv4 ACLs, this sort of situation would result in duplicated
entries.

MFC after: 1 month

Switch to our preferred license text.

Approved by: jedgar

Improve ACL branding mismatch detection and reporting in some rare cases,
such as "setfacl -m ''".

Add NFSv4 support to setfacl(1).

Reviewed by: rwatson

/*- or .\"- or #- to begin license clauses.

Consistently use FBSDID

Remove extraneous blank line

Move the code merging ACL_USER and ACL_GROUP objects from under
two nested while loops into a separate function.

Obtained from: TrustedBSD Project

Add defines for access and default ACLs (ACCESS_ACL/DEFAULT_ACL)
to enhance readability.

Obtained from: TrustedBSD Project

style(9) cleanups mostly consisting of:
o explicitly check return values and variables against a value
o return x; -> return (x);
o fix inconsistent sysexits usage by nuking it (partially
suggested by bde)

Obtained from: TrustedBSD Project

o Separate acl_t into internal and external representations as
required by POSIX.1e. This maintains the current 'struct acl'
in the kernel while providing the generic external acl_t
interface required to complete the ACL editing library.
o Add the acl_get_entry() function.
o Convert the existing ACL utilities, getfacl and setfacl, to
fully make use of the ACL editing library.

Obtained from: TrustedBSD Project

o POSIX.2c Userland tool support for POSIX.1e ACLs -- getfacl retrieves ACLs
from files and directories, and setfacl sets ACLs on files and directories.

Submitted by: jedgar
Obtained from: TrustedBSD Project