doc: fix formatting, add remaining issues
module: strip leading lines when -x switch set

Factor the code that removes '#!' into a function,
Module._removePrologue.

Module._removePrologue removes only the first #! line unless
BOTH a) this is the main module (module.id === '.')
 AND b) the '-x' switch was passed on the command line.

In case both a) and b) are true, Module._removePrologue
removes lines until it finds a line which begins with "#!"
and contains the substring "node".
command-line: add support for -x switch

Detect '-x' switch on command line and set global
strip_until_shebang_node and property
process._stripUntilShebangNode
PrintHelp: add documentation of -x switch

Add documentation of -x switch:

"-x  ignore text before #!node line"
module: (_removePrologue) fix js style errors
doc: indent code example further
doc: add section, fix EOL
module: stop stripping lines when #!node seen

Explicit 'true' return value omitted when refactoring.
node: -x switch; no optional <dir> arg

Remove incorrect documentation referring to optional <dir>
arg to proposed -x switch.
doc: fix indentation
Merge branch 'minus-x-switch' of https://github.com/smikes/node into minus-x-switch