Avoid unnecessary calls to vm_map_protect() in elf_load_section().

Typically, when elf_load_section() unconditionally passed VM_PROT_ALL to
elf_map_insert(), it was needlessly enabling execute access on the
mapping, and it would later have to call vm_map_protect() to correct the
mapping's access rights.  Now, instead, elf_load_section() always passes
its parameter "prot" to elf_map_insert().  So, elf_load_section() must
only call vm_map_protect() if it needs to remove the write access that
was temporarily granted to perform a copyout().