Removed the altmsglink (named `@') and env var `$editalt' for repl and dist.

The env var $mhaltmsg already provides everything. We just get rid of
compat redundancy.
We also dropped the lstat() check. lstat() is POSIX and we assume any
modern Unix system to support symbolic links. Also, there's only one
invocation of lstat() left in uip/flist.c.