TMPFILE support

TMPFILE support

Currently on DOSISH machines a -e line with a syntax error results in
persisting pl234523 tmpfile. The patch below:

a) Cleans up the cleanup code;
b) Moves the TMPPATH specifics to header files.

I do not send patches for OS/2 files, since they are very much
reworked now, I only note that b) allows OS/2 startup to specify
TMPPATH to look to a $ENV{TMP} directory (instead of the current
directory, as it is now). Note that this allows now running -e perl
from read-only media.

Since there is no place now to have DOSISH _code_, as opposed to
_headers_, I leave the TMPPATH code on ./os2/os2.c, though it should
be common to DOS as well.

Enjoy,
Ilya