Add some basic security policies with sensible defaults

This ommmit contains some security hardening measures for the Hugo build runtime.

There are some rarely used features in Hugo that would be good to have disabled by default. One example would be the "external helpers".

For `asciidoctor` and some others we use Go's `os/exec` package to start a new process.

These are a predefined set of binary names, all loaded from `PATH` and with a predefined set of arguments. Still, if you don't use `asciidoctor` in your project, you might as well have it turned off.

You can configure your own in the new `security` configuration section, but the defaults are configured to create a minimal amount of site breakage. And if that do happen, you will get clear instructions in the loa about what to do.

The default configuration is listed below. Note that almost all of these options are regular expression _whitelists_ (a string or a slice); the value `none` will block all.

```toml
[security]
  enableInlineShortcodes = false
  [security.exec]
    allow = ['^dart-sass-embedded$', '^go$', '^npx$', '^postcss$']
    osEnv = ['(?i)^(PATH|PATHEXT|APPDATA|TMP|TEMP|TERM)$']

  [security.funcs]
    getenv = ['^HUGO_']

  [security.http]
    methods = ['(?i)GET|POST']
    urls = ['.*']
```

1 files changed, 19 insertions, 23 deletions

diff --git a/hugolib/site.go b/hugolib/site.go
index 96cf0b93c..dce4b8d25 100644
--- a/hugolib/site.go
+++ b/hugolib/site.go
@@ -120,8 +120,6 @@ type Site struct {
 
 	disabledKinds map[string]bool
 
-	enableInlineShortcodes bool
-
 	// Output formats defined in site config per Page Kind, or some defaults
 	// if not set.
 	// Output formats defined in Page front matter will override these.
@@ -378,25 +376,24 @@ func (s *Site) isEnabled(kind string) bool {
 // reset returns a new Site prepared for rebuild.
 func (s *Site) reset() *Site {
 	return &Site{
-		Deps:                   s.Deps,
-		disabledKinds:          s.disabledKinds,
-		titleFunc:              s.titleFunc,
-		relatedDocsHandler:     s.relatedDocsHandler.Clone(),
-		siteRefLinker:          s.siteRefLinker,
-		outputFormats:          s.outputFormats,
-		rc:                     s.rc,
-		outputFormatsConfig:    s.outputFormatsConfig,
-		frontmatterHandler:     s.frontmatterHandler,
-		mediaTypesConfig:       s.mediaTypesConfig,
-		language:               s.language,
-		siteBucket:             s.siteBucket,
-		h:                      s.h,
-		publisher:              s.publisher,
-		siteConfigConfig:       s.siteConfigConfig,
-		enableInlineShortcodes: s.enableInlineShortcodes,
-		init:                   s.init,
-		PageCollections:        s.PageCollections,
-		siteCfg:                s.siteCfg,
+		Deps:                s.Deps,
+		disabledKinds:       s.disabledKinds,
+		titleFunc:           s.titleFunc,
+		relatedDocsHandler:  s.relatedDocsHandler.Clone(),
+		siteRefLinker:       s.siteRefLinker,
+		outputFormats:       s.outputFormats,
+		rc:                  s.rc,
+		outputFormatsConfig: s.outputFormatsConfig,
+		frontmatterHandler:  s.frontmatterHandler,
+		mediaTypesConfig:    s.mediaTypesConfig,
+		language:            s.language,
+		siteBucket:          s.siteBucket,
+		h:                   s.h,
+		publisher:           s.publisher,
+		siteConfigConfig:    s.siteConfigConfig,
+		init:                s.init,
+		PageCollections:     s.PageCollections,
+		siteCfg:             s.siteCfg,
 	}
 }
 
@@ -564,8 +561,7 @@ But this also means that your site configuration may not do what you expect. If
 		outputFormatsConfig: siteOutputFormatsConfig,
 		mediaTypesConfig:    siteMediaTypesConfig,
 
-		enableInlineShortcodes: cfg.Language.GetBool("enableInlineShortcodes"),
-		siteCfg:                siteConfig,
+		siteCfg: siteConfig,
 
 		titleFunc: titleFunc,