Add $&getpid primitive and use that to initialize $pid at startup.

This is actually an unrelated mailing list suggestion (see
http://wryun.github.io/es-shell/mail-archive/msg00977.html) which
just happens to work very well in the context of a %main hook.

Author: jpco

main.c

@@ -2,11 +2,6 @@
 
 #include "es.h"
 
-/* initpid -- set $pid for this shell */
-static void initpid(void) {
-	vardef("pid", NULL, mklist(mkstr(str("%d", getpid())), NULL));
-}
-
 /* main -- initialize, parse command arguments, and start running */
 int main(int argc, char **argv) {
 	initgc();
@@ -18,7 +13,6 @@ int main(int argc, char **argv) {
 
 	runinitial();
 
-	initpid();
 	initsignals();
 	hidevariables();
 	importenv(FALSE);

prim-etc.c

@@ -265,6 +265,10 @@ PRIM(importenvfuncs) {
 	return true;
 }
 
+PRIM(getpid) {
+	return mklist(mkstr(str("%d", getpid())), NULL);
+}
+
 #if READLINE
 PRIM(resetterminal) {
 	resetterminal = TRUE;
@@ -302,6 +306,7 @@ extern Dict *initprims_etc(Dict *primdict) {
 	X(noreturn);
 	X(setmaxevaldepth);
 	X(importenvfuncs);
+	X(getpid);
 #if READLINE
 	X(resetterminal);
 #endif

runtime.es

@@ -36,6 +36,7 @@ let (
 )
 fn %main argv {
 	fn-%main = ()
+	pid = <=$&getpid
 
 	let (
 		es = es