feat(process): fork/exec/id/session + system/popen

include/stdlib.h

@@ -463,6 +463,19 @@ mkstemp(char *);
 char *
 mkdtemp(char *);
 
+/* process control (todo 20) */
+
+/*
+ * Pass string to the command language interpreter: "sh -c string".
+ * Returns the shell's wait status (e.g. 768 for "exit 3"), 1 when
+ * string is NULL (a shell is always available), or -1 with errno set
+ * when the child cannot be created or reaped. During the run, SIGCHLD
+ * is blocked and SIGINT/SIGQUIT are ignored in the caller, as POSIX
+ * requires.
+ */
+int
+system(const char *string);
+
 #if VLIBC_LEVEL_GE(2)
 /* Level 2 (muslmimic): XSI and BSD environment extras. */