MFC r237058:

MFC r237058:
Eliminate the static buffer used to read the first page of the mapped
object, and eliminate the pread(2) call as well. Mmap the first
page of the object temporaly, and unmap it on error or last use.
Potentially, this leaves one-page gap between succeeding dlopen(3),
but there are other mmap(2) consumers as well.

Fix several cases were the whole mapping of the object leaked on error.

Use MAP_PREFAULT_READ for mmap(2) calls which map real object pages

git-svn-id: svn://svn.freebsd.org/base/stable/9@237712 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f