od: add double precision float dump

[mknos]

• GNU and OpenBSD versions of od support -e and -F for printing two 8-byte floating point numbers per line
• Write a new formatting function to handle this
• While here, add whitespace for readability of -f flag (regular float dump) and increase the precision digits to 7 to match GNU

[briandfoy]

changes: add -e and -f for printing two 8-byte floating point numbers per line