file.c: move umask parser to a separate file

* umask.c: New file.
* Makefile.am (strace_SOURCES): Add it.
* file.c (sys_umask): Move to umask.c.
diff --git a/Makefile.am b/Makefile.am
index 47b90a2..79f9b1b 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -79,6 +79,7 @@
 	term.c		\
 	time.c		\
 	truncate.c	\
+	umask.c		\
 	umount.c	\
 	util.c		\
 	utime.c		\