ts/kill/decode: use RTMIN from 'kill -L' instead of hardcoding 34

ts/kill/decode: use RTMIN from 'kill -L' instead of hardcoding 34

glibc uses 34 as the value of SIGRTMIN:
https://sourceware.org/git/?p=glibc.git;a=blob;f=signal/allocrtsig.c;h=8ed8e37dd6c41f94be6eef042ce9db1af1153228;hb=HEAD#l27 """
static int current_rtmin = __SIGRTMIN + RESERVED_SIGRT; """

musl uses 35 as the value of SIGRTMIN:
https://git.musl-libc.org/cgit/musl/tree/src/signal/sigrtmin.c

With the hardcoded 34, test case fails with the following difference:

-Ignored: HUP QUIT TRAP PIPE ALRM
+Ignored: HUP QUIT TRAP PIPE ALRM 34

Extract the value of RTMIN from 'kill -L' to avoid such hardcoding.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>