tweaks: constify the result strings of getenv(), as a reminder

Also, remove an unneeded pair of braces.

src/history.c

@@ -250,7 +250,7 @@ void history_error(const char *msg, ...)
 bool have_statedir(void)
 {
     struct stat dirstat;
-    char *xdgdatadir;
+    const char *xdgdatadir;
 
     get_homedir();
 
@@ -269,9 +269,9 @@ bool have_statedir(void)
     if (homedir == NULL && xdgdatadir == NULL)
 	return FALSE;
 
-    if (xdgdatadir != NULL) {
+    if (xdgdatadir != NULL)
 	statedir = concatenate(xdgdatadir, "/nano/");
-    } else
+    else
 	statedir = concatenate(homedir, XDG_DATA_FALLBACK "/nano/");
 
     if (stat(statedir, &dirstat) == -1) {

src/nano.c

@@ -2422,7 +2422,7 @@ int main(int argc, char **argv)
      * checking is disabled, since it would allow reading from or
      * writing to files not specified on the command line). */
     if (!ISSET(RESTRICTED) && alt_speller == NULL) {
-	char *spellenv = getenv("SPELL");
+	const char *spellenv = getenv("SPELL");
 	if (spellenv != NULL)
 	    alt_speller = mallocstrcpy(NULL, spellenv);
     }

src/rcfile.c

@@ -1215,7 +1215,7 @@ void parse_one_nanorc(void)
 	rcfile_error(N_("Error reading %s: %s"), nanorc, strerror(errno));
 }
 
-bool have_nanorc(char *path, char *name)
+bool have_nanorc(const char *path, char *name)
 {
     if (path == NULL)
 	return FALSE;
@@ -1229,6 +1229,8 @@ bool have_nanorc(char *path, char *name)
 /* First read the system-wide rcfile, then the user's rcfile. */
 void do_rcfiles(void)
 {
+    const char *xdgconfdir;
+
     nanorc = mallocstrcpy(nanorc, SYSCONFDIR "/nanorc");
 
     /* Process the system-wide nanorc. */
@@ -1242,8 +1244,7 @@ void do_rcfiles(void)
 #endif
 
     get_homedir();
-
-    char *xdgconfdir = getenv("XDG_CONFIG_HOME");
+    xdgconfdir = getenv("XDG_CONFIG_HOME");
 
     /* Now try the to find a nanorc file in the user's home directory
      * or in the XDG configuration directories. */

src/text.c

@@ -1108,7 +1108,7 @@ bool execute_command(const char *command)
 {
     int fd[2];
     FILE *f;
-    char *shellenv;
+    const char *shellenv;
     struct sigaction oldaction, newaction;
 	/* Original and temporary handlers for SIGINT. */
     bool sig_failed = FALSE;