In rtld's map_object(), use pread(..., 0) rather than read() to read the

ELF header from the front of the file.  As all other I/O on the binary
is done using mmap(), this avoids the need for seek privileges on the
file descriptor during run-time linking.

MFC after:	1 month
Sponsored by:	Google

libexec/rtld-elf/map_object.c

@@ -273,7 +273,7 @@ get_elf_header (int fd, const char *path)
     } u;
     ssize_t nbytes;
 
-    if ((nbytes = read(fd, u.buf, PAGE_SIZE)) == -1) {
+    if ((nbytes = pread(fd, u.buf, PAGE_SIZE, 0)) == -1) {
 	_rtld_error("%s: read error: %s", path, strerror(errno));
 	return NULL;
     }