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

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

Remove the advertising clause from UCB copyrighted files in usr.bin. This
is in accordance with the information provided at
ftp://ftp.cs.berkeley.edu/pub/4bsd/README.Impt.License.Change

Also add $FreeBSD$ to a few files to keep svn happy.

Discussed with: imp, rwatson

Update the date on the man-page to reflect the date that the '-u name'
change was committed, instead of when I had first started writing it...

Add the '-u name' option to the env command, which will completely unset
the given variable name (removing it from the environment, instead of
just setting it to a null value).

PR: bin/65649
MFC after: 2 weeks

Tidy up the markup.

Approved by: re (blanket)
OK'ed by: gad

Put in a more descriptive heading for the `env' command.

Noticed by: Matthew D Fuller
Approved by: re (blanket `env')

Undo r1.14 to env.c. While it is unusual for environment variables to
start with a '/', they are more supported (by POSIX and SUSv3) than
filenames which have an '=' in them.

Noticed by: tjr
Approved by: re (blanket `env')

Add the '-S' and '-P' options. The '-S' option can be used to split
apart a string, and supports some text substitutions. This can be
used to provide all the flexibility (and more!) that was lost by recent
changes to how the kernel parses #!-lines in shell scripts.

The '-P' option provides a way to specify an alternate set of directories
to use when searching for the 'utility' program to run. This way you can
be sure what directories are used for that search, without changing the
value of PATH that the user has set. Note that on FreeBSD 6.0, this
option is worthless unless the '-S' option is also used.

Approved by: re (blanket `env')

Added the EXIT STATUS section where appropriate.

Language and markup polishing.

Fix grammar bogon.

Removed (after the env(1)/printenv(1) split) extraneous arguments
to the .Nm macro.

Split the env(1) manpage off from printenv(1); there is not much point
in keeping them together. Mention that printenv is obsolescent.