MFC r345661:

Factor out retrieving the interpreter path from the main ELF
loader routine.

MFC r345734 by kib:

Fix branding after r345661.

In particular, elf32 FreeBSD binaries were not executed on LP64 hosts.
The interp_name_len value should account for the nul terminator. This
is needed for strncmp()s in brand checking code to work.

Sponsored by: DARPA, AFRL