make timeout(1) -s behave like kill(1) -s and GNU timeout(1) -s

[Klemens Nanni]

timeout.c's parse_signal() basically does what kill.c's signame_to_num()
does, except it expects "SIG" in string signals.

Borrow the isalpha() check from kill.c to get the same behaviour:

        $ time ./obj/timeout -s kill 1 sleep 3
        Killed 
            0m01.01s real     0m00.00s user     0m00.01s system
        $ time timeout -s kill 1 sleep 3      
        timeout: signal kill invalid
        0m00.00s real     0m00.00s user     0m00.00s system

Feedback? OK?

Index: timeout.c
===================================================================
RCS file: /cvs/src/usr.bin/timeout/timeout.c,v
retrieving revision 1.20
diff -u -p -r1.20 timeout.c
--- timeout.c   12 Jan 2022 22:51:44 -0000      1.20
+++ timeout.c   2 Jul 2022 07:13:45 -0000
@@ -32,6 +32,7 @@
 #include <sys/time.h>
 #include <sys/wait.h>
 
+#include <ctype.h>
 #include <err.h>
 #include <errno.h>
 #include <getopt.h>
@@ -104,10 +105,11 @@ parse_signal(const char *str)
        long long        sig;
        const char      *errstr;
 
-       if (strncasecmp(str, "SIG", 3) == 0) {
+       if (isalpha((unsigned char)*str)) {
                int i;
 
-               str += 3;
+               if (strncasecmp(str, "SIG", 3) == 0)
+                       str += 3;
                for (i = 1; i < NSIG; i++) {
                        if (strcasecmp(str, sys_signame[i]) == 0)
                                return (i);