Support the -c and -g options.

These allow one to select the system calls to fuzz. If a system call
matches either option, it will be fuzzed. Example usages:

 $ sysfuzz -g sched # Fuzz all scheduler-related system calls.

 $ sysfuzz -c mprotect,mlock # Fuzz mprotect(2) and mlock(2) only.

 $ sysfuzz -g vm -c fork # Fuzz the mmap(2) calls and fork(2).