Change _p9strsig to return nil if tmp==nil.

Add atoi, atol, atoll.
These versions call strtol/strtoll with base==0.
The Unix versions use base==10.
diff --git a/src/lib9/mkfile b/src/lib9/mkfile
index 4dda2e1..6dfab74 100644
--- a/src/lib9/mkfile
+++ b/src/lib9/mkfile
@@ -72,6 +72,9 @@
 	announce.$O\
 	argv0.$O\
 	atexit.$O\
+	atoi.$O\
+	atol.$O\
+	atoll.$O\
 	atnotify.$O\
 	await.$O\
 	cistrcmp.$O\