unshare: fix SYNOPSIS and usage()

The "program" is optional and $SHELL is executed by default.

Addresses: https://github.com/karelzak/util-linux/issues/389
Signed-off-by: Karel Zak <kzak@redhat.com>

sys-utils/unshare.1

@@ -4,11 +4,12 @@ unshare \- run program with some namespaces unshared from parent
 .SH SYNOPSIS
 .B unshare
 [options]
-.I program
-.RI [ arguments ]
+.RI [ program
+.RI [ arguments ]]
 .SH DESCRIPTION
 Unshares the indicated namespaces from the parent process and then executes
-the specified \fIprogram\fR.
+the specified \fIprogram\fR. If \fIprogram\fR is not given, then ``${SHELL}'' is
+run (default: /bin/sh).
 .PP
 The namespaces can optionally be made persistent by bind mounting
 /proc/\fIpid\fR/ns/\fItype\fR files to a filesystem path and entered with

sys-utils/unshare.c

@@ -243,7 +243,7 @@ static void usage(int status)
 	FILE *out = status == EXIT_SUCCESS ? stdout : stderr;
 
 	fputs(USAGE_HEADER, out);
-	fprintf(out, _(" %s [options] <program> [<argument>...]\n"),
+	fprintf(out, _(" %s [options] [<program> [<argument>...]]\n"),
 		program_invocation_short_name);
 
 	fputs(USAGE_SEPARATOR, out);