makesyscalls: rip out arbitrary command execution

This was previously needed only for CloudABI, which used it to generate
its capenabled from syscalls.master.  CloudABI was removed in
cf0ee8738e, so we don't need to support this anymore.  Others looking
to do similar things should come up with a more integrated technique,
such as a .conf flag or pattern/glob support.  brooks suggests that it
could be done in modern makesyscalls.lua by adding a config flag to
specify always-on/initial flags (CAPENABLED).

Reviewed by:	brooks, imp
MFC after:	never
Differential Revision:	https://reviews.freebsd.org/D32095

sys/tools/makesyscalls.lua

@@ -195,19 +195,6 @@ local function trim(s, char)
 	return s:gsub("^" .. char .. "+", ""):gsub(char .. "+$", "")
 end
 
--- We have to io.popen it, making sure it's properly escaped, and grab the
--- output from the handle returned.
-local function exec(cmd)
-	cmd = cmd:gsub('"', '\\"')
-
-	local shcmd = "/bin/sh -c \"" .. cmd .. "\""
-	local fh = io.popen(shcmd)
-	local output = fh:read("a")
-
-	fh:close()
-	return output
-end
-
 -- config looks like a shell script; in fact, the previous makesyscalls.sh
 -- script actually sourced it in.  It had a pretty common format, so we should
 -- be fine to make various assumptions
@@ -240,8 +227,9 @@ local function process_config(file)
 			key = trim(key)
 			value = trim(value)
 			local delim = value:sub(1,1)
-			if delim == '`' or delim == '"' then
+			if delim == '"' then
 				local trailing_context
+
 				-- Strip off the key/value part
 				trailing_context = nextline:sub(kvp:len() + 1)
 				-- Strip off any trailing comment
@@ -253,26 +241,7 @@ local function process_config(file)
 					print(trailing_context)
 					abort(1, "Malformed line: " .. nextline)
 				end
-			end
-			if delim == '`' then
-				-- Command substition may use $1 and $2 to mean
-				-- the syscall definition file and itself
-				-- respectively.  We'll go ahead and replace
-				-- $[0-9] with respective arg in case we want to
-				-- expand this in the future easily...
-				value = trim(value, delim)
-				for capture in value:gmatch("$([0-9]+)") do
-					capture = tonumber(capture)
-					if capture > #arg then
-						abort(1, "Not enough args: " ..
-						    value)
-					end
-					value = value:gsub("$" .. capture,
-					    arg[capture])
-				end
 
-				value = exec(value)
-			elseif delim == '"' then
 				value = trim(value, delim)
 			else
 				-- Strip off potential comments