MFC r338262, r339334, r339796, r340240, r340857, r340917, r341007

r338262:
stand: fdt: Drop some write-only assignments/variables and leaked bits

Generally straightforward enough; a copy of argv[1] was being made in
command_fdt_internal, solely used for a comparison within the
handler-search, then promptly leaked.

r339334:
loader.efi: add poweroff command

Add poweroff command to make life a bit easier.

r339796:
Simplify the EFI delay() function by calling BS->Stall()

r340240:
loader: ptable_open() check for ptable_cd9660read result is wrong

The ptable_*read() functions return NULL on read errors (and partition table
closed as an side effect). The ptable_open must check the return value and
act properly.

r340857:
Nuke out buffer overflow safety marker code, it duplicates similar code in
the malloc()/free() as well as having potential of softening the handling
in case error is detected down to a mere warning as compared to hard panic
in free().

r340917:
Update pxeboot(8) manual page to reflect the next-server change in the ISC DHCP v3 server.

r341007:
Bump the date of pxeboot(8) manual page for r340917.

PR: 123484, 232483