functional-differences: system() thread safety

Done as issue requests. Closes #63.

functional-differences-from-glibc.md

@@ -370,10 +370,12 @@ musl also does not implement the following glibc bugs:
 
 # Miscellaneous functions with GNU quirks
 
-- GNU getopt permutes argv to pull options to the front, ahead of non-option
-  arguments. musl and the POSIX standard getopt stop processing options at the
+- GNU `getopt` permutes argv to pull options to the front, ahead of non-option
+  arguments. musl and the POSIX standard `getopt` stop processing options at the
   first non-option argument with no permutation.
-- glibc provides two versions of basename. The one declared in stdlib.h has
+- glibc provides two versions of `basename`. The one declared in stdlib.h has
   alternate semantics and signature that conflict with the standard. musl only
   provides the standard one.
+- glibc provides a thread-safe `system`. Thread safety for `system` is not 
+  required by POSIX and musl's version is not thread-safe.