Standardize usage string var declarations

Changes:

* Name them all "usage_str"
* Make them const

For the latter item, see commit eb20f52 ("Make list of paths const to
fix a false positive of gcc analyzer", 2022-07-27) / PR netblue30#5275.

src/firecfg/main.c

@@ -24,7 +24,7 @@ int arg_debug = 0;
 char *arg_bindir = "/usr/local/bin";
 int arg_guide = 0;
 
-static char *usage_str =
+static const char *const usage_str =
 	"Firecfg is the desktop configuration utility for Firejail software. The utility\n"
 	"creates several symbolic links to firejail executable. This allows the user to\n"
 	"sandbox applications automatically, just by clicking on a regular desktop\n"

src/firejail/usage.c

@@ -19,7 +19,7 @@
 */
 #include "firejail.h"
 
-static char *usage_str =
+static const char *const usage_str =
 	"Firejail is a SUID sandbox program that reduces the risk of security breaches by\n"
 	"restricting the running environment of untrusted applications using Linux\n"
 	"namespaces.\n"

src/firemon/usage.c

@@ -19,7 +19,7 @@
 */
 #include "firemon.h"
 
-static char *help_str =
+static const char *const usage_str =
 	"Usage: firemon [OPTIONS] [PID]\n\n"
 	"Monitor processes started in a Firejail sandbox. Without any PID specified,\n"
 	"all processes started by Firejail are monitored. Descendants of these processes\n"
@@ -84,5 +84,5 @@ void print_version(void) {
 
 void usage(void) {
 	print_version();
-	puts(help_str);
+	puts(usage_str);
 }

src/jailcheck/main.c

@@ -29,7 +29,7 @@ char *user_home_dir = NULL;
 char *user_run_dir = NULL;
 int arg_debug = 0;
 
-static char *usage_str =
+static const char *const usage_str =
 	"Usage: jailcheck [options] directory [directory]\n\n"
 	"Options:\n"
 	"   --debug - print debug messages.\n"