diff3: prefer posix_spawn over pdfork/execlp

[kib]

Unrelated note: it seems that pipe fds leak to both pd13 and pd23 processes. This is perhaps not too important but just not accurate. And since we care about 'security' by entering cap mode, ability of the children to affect input of siblings or parent might be considered undesirable.

usr.bin/diff3/diff3.c

1092

Spurious blank line