[PATCH] add exec -a support (preliminary)

[Patrick Pief]

I just noticed that I have a little bug in there, triggered through
"exec -a foo" with no parameter following, it can be fixed by replacing the
line

    if (strcmp(argv[1], "-a") == 0 && argv[2]) {

with

    if (strcmp(argv[1], "-a") == 0 && argv[2] && argv[3]) {

to check whether an argument after "-a something" exists.

-Patrick