MFC r307927

MFC r307927

 Be more precise when including headers so that we're less likely to
 depend on namespace pollution and as such become more portable. This
 means including headers like <sys/types.h> or <stdlib.h>, but also
 making sure we include system/host headers before local headers.

 While here: define ENOATTR as ENOMSG in mtree.c. There is no ENOATTR
 on Linux.

 With this, makefs is ready for compilation on macOS and Linux.

Sponsored by:	iXsystems, Inc.