genhelp.lua: don't match underscores in the middle of words

Fixes help of mkndx and fetch which printed PKGNAMESPEC_ in the help
output. This also matches the behaviour of scdoc.

src/genhelp.lua

@@ -138,8 +138,10 @@ function scdoc:parse_default(ln)
 	-- Handle formatting
 	ln = ln:gsub("([^\\])%*(.-[^\\])%*", "%1%2")
 	ln = ln:gsub("^%*(.-[^\\])%*", "%1")
-	ln = ln:gsub("([^\\])_(.-[^\\])_", function(a,s) return a..s:upper() end)
-	ln = ln:gsub("^_(.-[^\\])_", function(s) return s:upper() end)
+	ln = ln:gsub("([^\\a-zA-Z0-9])_(.-[^\\])_([^a-zA-Z0-9])", function(a,s,e) return a..s:upper()..e end)
+	ln = ln:gsub("([^\\a-zA-Z0-9])_(.-[^\\])_$", function(a,s) return a..s:upper() end)
+	ln = ln:gsub("^_(.-[^\\])_([^a-zA-Z0-9])", function(s,e) return s:upper()..e end)
+	ln = ln:gsub("^_(.-[^\\])_$", function(s) return s:upper() end)
 	ln = ln:gsub("\\", "")
 
 	self:section_text(ln)