kill: split out from skill/snice

The first part of fixing skill/snice to use the library instead
of directly readdir()ing /proc which is what it does now.

Remove the kill code from the skill/snice code and put common
elements into lib/signals.c Not 100% sure that is the right
destination instead of a new lib file, but ok for now.

kill shares some parsing logic with skill/snice but mainly
around signal specifications. The "do it" code is very different.

Signed-off-by: Craig Small <csmall@enc.com.au>

include/signals.h

@@ -24,5 +24,13 @@ extern int signal_name_to_number(const char *__restrict name);
 
 extern const char *signal_number_to_name(int signo);
 
+extern int skill_sig_option(int *argc, char **argv);
+
+extern char *strtosig(const char *restrict s);
+
+extern void pretty_print_signals(void);
+
+extern void unix_print_signals(void);
+
 __END_DECLS
 #endif