Add a way to mark environment variables as "hidden" so they can be used

by tmux but are not passed into the environment of new panes.

tmux.1

@@ -565,6 +565,18 @@ Environment variables may be set by using the syntax
 for example
 .Ql HOME=/home/user .
 Variables set during parsing are added to the global environment.
+A hidden variable may be set with
+.Ql %hidden ,
+for example:
+.Bd -literal -offset indent
+%hidden MYVAR=42
+.Ed
+.Pp
+Hidden variables are not passed to the environment of processes created
+by tmux.
+See the
+.Sx GLOBAL AND SESSION ENVIRONMENT
+section.
 .Pp
 Commands may be parsed conditionally by surrounding them with
 .Ql %if ,
@@ -4711,10 +4723,16 @@ from inside, and the
 variable with the correct terminal setting of
 .Ql screen .
 .Pp
+Variables in both session and global environments may be marked as hidden.
+Hidden variables are not passed into the environment of new processes and
+instead can only be used by tmux itself (for example in formats, see the
+.Sx FORMATS
+section).
+.Pp
 Commands to alter and view the environment are:
 .Bl -tag -width Ds
 .It Xo Ic set-environment
-.Op Fl gru
+.Op Fl hgru
 .Op Fl t Ar target-session
 .Ar name Op Ar value
 .Xc
@@ -4731,8 +4749,10 @@ flag unsets a variable.
 .Fl r
 indicates the variable is to be removed from the environment before starting a
 new process.
+.Fl h
+marks the variable as hidden.
 .It Xo Ic show-environment
-.Op Fl gs
+.Op Fl hgs
 .Op Fl t Ar target-session
 .Op Ar variable
 .Xc
@@ -4749,6 +4769,8 @@ Variables removed from the environment are prefixed with
 If
 .Fl s
 is used, the output is formatted as a set of Bourne shell commands.
+.Fl h
+shows hidden variables (omitted by default).
 .El
 .Sh STATUS LINE
 .Nm