convert file header comment to docstring performing same function

also usage examples into docstring, but copyright in comment above for no
reason
imports on own lines (pep8 tool: E401)
remove unused 'os' import
blank lines between functions (pep8)
spacing around operators, commas, etc. (pep8)
2-space to 4-space tab size (pep8)
remove unused 'encoding' var in func_hexdump
remove unused 'byteseq' var in func_output
create main(), push parser creation into own function
change app_version to __version__

pointless but a little more idiomatic to python
remove nargs=1 in places where we don't want a list anyway
remove parentheses around conditions in if statements
meh, comment tweak
only call sys.exit at one point in program, get status from func_* returns

reason:
tracking control flow of programs with lots of sys.exit can get nasty
when things like context managers and exception handling get involved...

if func_* does not have a return statement or returns None/falsy,
OS will get 0 (success)
streamline checking and printing args.encoding (hexdump, output)
don't keep working after calling usage() but return
start DRYing up redundant subparser setups
let argparse generate semi-meaningful usage message

may customize later
oops: wasn't handling case where func was not set
continue DRYing up subparser declarations