fileutil: Omit zsh extensions and shebangs

[dcarley]

testdata: Fix missing zsh files

These lines were being rendered into the contents of shebang-nospace rather than separate files because they were missing the trailing -- delimiters.

It seems that there's no way to guard against this based txtar docs:

There are no possible syntax errors in a txtar archive.

This change will cause the tests to fail and will be fixed in a subsequent commit:

> cmpenv stdout find.golden
 diff stdout find.golden
 --- stdout
 +++ find.golden
 @@ -16,6 +16,4 @@
  modify/shebang-space
  modify/shebang-tabs
  modify/shebang-usr-sh
 -skip/ext.zsh
 -skip/shebang-zsh
  symlink/subdir/ext-shebang.sh

 FAIL: testdata/script/walk.txtar:10: stdout and find.golden differ

fileutil: Omit zsh extensions and shebangs

To prevent shfmt from discovering zsh files which aren't currently supported for formatting.

This fixes #535 and the following test:

> cmpenv stdout find.golden
 diff stdout find.golden
 --- stdout
 +++ find.golden
 @@ -16,6 +16,4 @@
  modify/shebang-space
  modify/shebang-tabs
  modify/shebang-usr-sh
 -skip/ext.zsh
 -skip/shebang-zsh
  symlink/subdir/ext-shebang.sh

 FAIL: testdata/script/walk.txtar:10: stdout and find.golden differ

It could be considered a breaking change to shfmt and fileutil.

There's some discussion in that issue about whether it's beneficial to match zsh files because they can sometimes be formatted if they don't contain any zsh-specific syntax, but without any way to explicitly exclude them from discovery it's more of a hindrance than help until #120 is done.