Rename mail(1)'s aux.c to util.c.

The name `aux' is reserved on Windows file systems. aux.c in the mail(1)
directory contains some random utility functions. I'm renaming this file
to util.c to make it possible to check out this directory on Windows.

MFC after:	1 month

usr.bin/mail/Makefile

@@ -2,9 +2,9 @@
 # $FreeBSD$
 
 PROG=	mail
-SRCS=	version.c aux.c cmd1.c cmd2.c cmd3.c cmdtab.c collect.c edit.c fio.c \
+SRCS=	version.c cmd1.c cmd2.c cmd3.c cmdtab.c collect.c edit.c fio.c \
 	getname.c head.c v7.local.c lex.c list.c main.c names.c popen.c \
-	quit.c send.c strings.c temp.c tty.c vars.c
+	quit.c send.c strings.c temp.c tty.c util.c vars.c
 FILES=	mail.help mail.tildehelp
 FILESDIR=	${SHAREDIR}/misc
 EFILES=	mail.rc