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/CMakeLists.txt

@@ -11,6 +11,7 @@ set(LIBC_SOURCES
     fnmatch.cpp
     ifaddrs.cpp
     getopt.cpp
+    getsubopt.cpp
     grp.cpp
     inttypes.cpp
     ioctl.cpp