lib/tst_sig.c: output signal name when got unexpected signal

When testcase is killed by unexpected signal, usually it just prints
signal's value.
	fcntl30     1  TBROK  :  unexpected signal 2 received (pid = 6714).
	fcntl30     2  TBROK  :  Remaining cases broken

Here we also print signal's name to output more informative message.
	fcntl30     1  TBROK  :  unexpected signal SIGINT(2) received (pid = 9872).
	fcntl30     2  TBROK  :  Remaining cases broken

In this patch, we also add two new tst_ functions to convert errno or signal
to its name. The prototype is below:
    const char *tst_strsig(int sig);   returns signal name given signal number
    const char *tst_strerrno(int err); returns errno name given errno number

Signed-off-by: Xiaoguang Wang <wangxg.fnst@cn.fujitsu.com>
Signed-off-by: Cyril Hrubis <chrubis@suse.cz>