extractScriptMetadata.sh: replace sed | awk with awk

Avoid the common antipattern [1] and reimplement the entire script in Awk instead.

[1] http://www.iki.fi/era/unix/award.html#grep

extractScriptMetadata.sh

@@ -1,2 +1,2 @@
 #!/bin/bash
-sed -e '/./{H;$!d;}' -e 'x;/UserScript/!d;' $1 | awk '!/^$/' > ${1/.user./.meta.}
+awk '/./; /\/UserScript/ { exit }' "$1" > "${1/.user./.meta.}"