Simplify and rename mmap_full_file().

There is only one caller which maps the file privately into read-only
memory. Moreover, this caller does not care about the file descriptor
from which the mapping was created.

This commit removes the argument from mmap_full_file() which control
the type of the map and the result pointer for the file descriptor,
thereby simplifying the function slightly. Rename it to mmap_file_ro()
to indicate that read-only private mappings are implied.