Delete the last emulation callbacks: we're Just ELF, so declare

Delete the last emulation callbacks: we're Just ELF, so declare
exec_elf_fixup() and coredump_elf() in <sys/exec_elf.h> and call
them and the MD setregs() directly in kern_exec.c and kern_sig.c

Also delete e_name[] (only used by sysctl), e_errno (unused), and
e_syscallnames[] (only used by SYSCALL_DEBUG) and constipate
syscallnames to 'const char *const[]'

ok kettenis@