LibC: Implement usleep()

This is not a POSIX function, but some ports seem to be using it either
way

userspace/libraries/LibC/include/sys/types.h

@@ -185,6 +185,12 @@ __BEGIN_DECLS
 #endif
 #undef __need_uid_t
 
+#if !defined(__useconds_t_defined) && (defined(__need_all_types) || defined(__need_useconds_t))
+	#define __useconds_t_defined 1
+	typedef unsigned long useconds_t;
+#endif
+#undef __need_useconds_t
+
 #ifdef __need_all_types
 	#include <stdint.h>
 #endif

userspace/libraries/LibC/include/unistd.h

@@ -101,6 +101,7 @@ __BEGIN_DECLS
 #define __need_gid_t
 #define __need_off_t
 #define __need_pid_t
+#define __need_useconds_t
 #include <sys/types.h>
 
 // FIXME: _CS prefixed definitions
@@ -206,6 +207,7 @@ char*				ttyname(int fildes);
 int					ttyname_r(int fildes, char* name, size_t namesize);
 int					unlink(const char* path);
 int					unlinkat(int fd, const char* path, int flag);
+int					usleep(useconds_t usec);
 ssize_t				write(int fildes, const void* buf, size_t nbyte);
 
 extern char*	optarg;

userspace/libraries/LibC/unistd.cpp

@@ -280,6 +280,14 @@ unsigned int sleep(unsigned int seconds)
 	return ret;
 }
 
+int usleep(useconds_t usec)
+{
+	timespec ts;
+	ts.tv_sec = usec / 1'000'000;
+	ts.tv_nsec = (usec % 1'000'000) * 1000;
+	return nanosleep(&ts, nullptr);
+}
+
 char* getcwd(char* buf, size_t size)
 {
 	if (size == 0)