MFC r345998-r346002, r346007-r346008: various loader improvements

r345998:
loader: malloc+bzero is calloc

Replace malloc+bzero in module.c with calloc.

r345999:
loader: file_addmodule should check for memory allocation

strdup() can return NULL.

r346000:
loader: remove pointer checks before free() in module.c

free() does check for NULL argument, remove duplicate checks.

r346001:
loader: file_addmetadata() should check for memory allocation

malloc() can return NULL.

r346002:
loader: mod_loadkld() error: we previously assumed 'last_file' could be null

The last_file variable is used to reset the loadaddr variable back to
original
value; however, it is possible the last_file is NULL, so we can not blindly
trust it. But then again, we can just save the original loadaddr and use
the saved value for recovery.

r346007:
loader: add file_remove() function to undo file_insert_tail().

346002 did miss the fact that we do not only undo the loadaddr, but also
we need to remove the inserted module. Implement file_remove() to do the
job.

r346008:
loader: command_lsefi: ret can be used uninitialized