LibC: Implement the getsubopt function

This is a LibC function that POSIX defines to help userspace programs
to get suboptions. An example of a suboption is the token "pixclk" from
a Shell command running "edid-decode --gtf w=1024,h=768,pixclk=48".
The function should be run in a while loop to acquire all suboptions
until the last one.

Userland/Libraries/LibC/unistd.h

@@ -167,5 +167,6 @@ extern int optreset;
 extern char* optarg;
 
 int getopt(int argc, char* const* argv, char const* short_options);
+int getsubopt(char** optionp, char* const* tokens, char** valuep);
 
 __END_DECLS