libproc-ng: add strtosig() function

The function will convert a signal number string to a signal name, or
vice a verse.  Return value is string, which is an function user is
expected to free after use.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>

proc/sig.h

@@ -22,6 +22,8 @@ extern const char *signal_number_to_name(int signo);
 
 extern int print_given_signals(int argc, const char *restrict const *restrict argv, int max_line);
 
+extern char *strtosig(const char *restrict s);
+
 extern void pretty_print_signals(void);
 
 extern void unix_print_signals(void);