Improve system calls

- Wrap clock_getres()
- Wrap sched_setscheduler()
- Make sleep() api conformant
- Polyfill sleep() using select()
- Improve clock_gettime() polyfill
- Make nanosleep() POSIX conformant
- Slightly improve some DNS functions
- Further strengthen pledge() sandboxing
- Improve rounding of timeval / timespec
- Allow layering of pledge() calls on Linux
- Polyfill sched_yield() using select() on XNU
- Delete more system constants we probably don't need

libc/dns/ent.h

@@ -1,11 +1,15 @@
 #ifndef COSMOPOLITAN_LIBC_DNS_ENT_H_
 #define COSMOPOLITAN_LIBC_DNS_ENT_H_
 #include "libc/dns/dns.h"
+
+#define HOST_NOT_FOUND 1
+#define TRY_AGAIN      2
+#define NO_RECOVERY    3
+#define NO_DATA        4
+
 #if !(__ASSEMBLER__ + __LINKER__ + 0)
 COSMOPOLITAN_C_START_
 
-extern int h_errno;
-
 struct netent {
   char *n_name;     /* official network name */
   char **n_aliases; /* alias list */
@@ -35,6 +39,10 @@ struct servent {
   char *s_proto;    /* protocol to use */
 };
 
+extern int h_errno;
+void herror(const char *);
+const char *hstrerror(int);
+
 struct netent *getnetent(void);
 struct netent *getnetbyname(const char *);
 struct netent *getnetbyaddr(uint32_t, int);