PR18885: Use /* unmodified-fnargs */ in tapsets

PR18885: Use /* unmodified-fnargs */ in tapsets

* runtime/stp_string.[ch]: Make _stp_text_str's "in" parameter const.
* tapset/dyninst/context-envvar.stp: Optimize env_var().
* tapset/linux/context-caller.stp: Optimize _caller_match().
* tapset/linux/kretprobe.stp: Optimize _set_kretprobe_string().
* tapset/linux/panic.stp: Speed up the panic().
* tapset/linux/rlimit.stp: Optimize rlimit_from_str().
* tapset/string.stp: Optimize strlen(), substr(), stringat(), isinstr(),
	text_str(), text_strn(), strtol(), isdigit().
* tapset/tokenize.stp: Optimize tokenize().
* tapset/uconversions.stp: Optimize user_string_n2(), user_string2_n_warn().