syscall: fix a few Linux system calls

These functions claimed to return error (an interface)
and be implemented entirely in assembly, but it's not
possible to create an interface from assembly
(at least not easily).

In reality the functions were written to return an errno uintptr
despite the Go prototype saying error.
When the errno was 0, they coincidentally filled out a nil error
by writing the 0 to the type word of the interface.
If the errno was ever non-zero, the functions would
create a non-nil error that would crash when trying to
call err.Error().

Luckily these functions (Seek, Time, Gettimeofday) pretty
much never fail, so it was all kind of working.

Found by go vet.

LGTM=bradfitz, r
R=golang-codereviews, bradfitz, r
CC=golang-codereviews
https://golang.org/cl/99320043

src/pkg/syscall/syscall_linux_386.go

@@ -135,7 +135,15 @@ func Setrlimit(resource int, rlim *Rlimit) (err error) {
 
 // Underlying system call writes to newoffset via pointer.
 // Implemented in assembly to avoid allocation.
-func Seek(fd int, offset int64, whence int) (newoffset int64, err error)
+func seek(fd int, offset int64, whence int) (newoffset int64, err Errno)
+
+func Seek(fd int, offset int64, whence int) (newoffset int64, err error) {
+	newoffset, errno := seek(fd, offset, whence)
+	if errno != 0 {
+		return 0, errno
+	}
+	return newoffset, nil
+}
 
 // Vsyscalls on amd64.
 //sysnb	Gettimeofday(tv *Timeval) (err error)