Use sh.vim local vars not global POSIX hacks

Rather than setting g:is_posix and working around core syntax/sh.vim's
ideas about Korn and POSIX shells, forego sh.vim's efforts to guess what
shell the system /bin/sh is entirely. It's irrelevant to me anyway,
since I'll often be writing shell scripts to run on an entirely
different system.

Instead, if we have a #!/bin/sh shebang reflected in the b:is_sh
variable set by core filetype.vim, and we don't have any other
buffer-level indication of what shell this is, assume it's POSIX,
because I very rarely write Bourne.

Then, after the syntax file is loaded, clear away all but one of the
resulting b:is_* variables.

I have a feeling this is going to end with me re-implementing this
syntax file, possibly as separate sh.vim, bash.vim, and ksh.vim files.

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