util.c: add and use ascii-only char functions

The "invalid_name" function claims to "allow strict ASCII letters and
numbers".

However, it uses isalnum(3) and isdigit(3), which may take the current
locale into account and thus return 1 for non-ASCII characters.

So add the following functions:

* ascii_isalnum
* ascii_isalpha
* ascii_isdigit
* ascii_islower
* ascii_isupper
* ascii_isxdigit

And use the applicable ones in "invalid_name" so that it actually uses
strictly ASCII in its comparisons.

Added on commit b4ffaa2 ("merges; more on cleaning up esc chars",
2023-02-14).

Relates to #5578.

Kind of relates to #5708.

src/firejail/firejail.h

@@ -580,6 +580,12 @@ int has_handler(pid_t pid, int signal);
 void enter_network_namespace(pid_t pid);
 int read_pid(const char *name, pid_t *pid);
 pid_t require_pid(const char *name);
+int ascii_isalnum(unsigned char c);
+int ascii_isalpha(unsigned char c);
+int ascii_isdigit(unsigned char c);
+int ascii_islower(unsigned char c);
+int ascii_isupper(unsigned char c);
+int ascii_isxdigit(unsigned char c);
 int invalid_name(const char *name);
 void check_homedir(const char *dir);
 

src/firejail/util.c

@@ -1448,15 +1448,42 @@ static int has_link(const char *dir) {
 	return 0;
 }
 
+int ascii_isalnum(unsigned char c) {
+	return (ascii_isalpha(c) || ascii_isdigit(c));
+}
+
+int ascii_isalpha(unsigned char c) {
+	return (ascii_islower(c) || ascii_isupper(c));
+}
+
+int ascii_isdigit(unsigned char c) {
+	return (c >= '0' && c <= '9');
+}
+
+int ascii_islower(unsigned char c) {
+	return (c >= 'a' && c <= 'z');
+}
+
+int ascii_isupper(unsigned char c) {
+	return (c >= 'A' && c <= 'Z');
+}
+
+int ascii_isxdigit(unsigned char c) {
+	int ret = (ascii_isdigit(c) ||
+	          (c >= 'a' && c <= 'f') ||
+	          (c >= 'A' && c <= 'F'));
+	return ret;
+}
+
 // allow strict ASCII letters and numbers; names with only numbers are rejected; spaces are rejected
 int invalid_name(const char *name) {
 	const char *c = name;
 
 	int only_numbers = 1;
 	while (*c) {
-		if (!isalnum(*c))
+		if (!ascii_isalnum(*c))
 			return 1;
-		if (!isdigit(*c))
+		if (!ascii_isdigit(*c))
 			only_numbers = 0;
 		++c;
 	}