Extend str_isspace to consider \r as whitespace, add documentation

This allows the function to be reused in more places where `\r` is also considered as whitespace.

src/base/system.cpp

@@ -3369,7 +3369,10 @@ void net_stats(NETSTATS *stats_inout)
 	*stats_inout = network_stats;
 }
 
-int str_isspace(char c) { return c == ' ' || c == '\n' || c == '\t'; }
+int str_isspace(char c)
+{
+	return c == ' ' || c == '\n' || c == '\r' || c == '\t';
+}
 
 char str_uppercase(char c)
 {

src/base/system.h

@@ -2029,7 +2029,20 @@ int str_toint(const char *str);
 int str_toint_base(const char *str, int base);
 unsigned long str_toulong_base(const char *str, int base);
 float str_tofloat(const char *str);
+
+/**
+ * Determines whether a character is whitespace.
+ *
+ * @ingroup Strings
+ *
+ * @param c the character to check
+ *
+ * @return 1 if the character is whitespace, 0 otherwise.
+ *
+ * @remark The following characters are considered whitespace: ' ', '\n', '\r', '\t'
+ */
 int str_isspace(char c);
+
 char str_uppercase(char c);
 int str_isallnum(const char *str);
 unsigned str_quickhash(const char *str);