Dryrun keyless ignore config

[gperciva]

Addresses #89

I went back and forth half a dozen times between handling fromconffile in load_keys() vs. dooption(). In the end, I went with load_keys() since dooption() is already pretty long, and each option isn't very long. But I wimped out on doing everything in load_keys(); I left bsdtar->have_keys = 1 in dooption() since that's where it was originally. I think it might be better to move that into load_keys(); if we did that, load_keys() could go back to being void instead of the int that I changed it to.

Basically: adding the functionality was trivial. Trying to make it match the desired style was not. :|

[cperciva]

This seems like it's begging for a if (errno == ENOENT) goto notfound; ?

[gperciva]

what about

[EACCES]
Search permission is denied for a component of the path prefix.

I mean, should that also count as file-not-found?

[gperciva]

Updated with cleaned-up code. Behaviour is now:

td@gin: ~/src/tarsnap/build (dryrun-keyless-ignore-config)
$ ./tarsnap -c -f a0 --dry-run ~/backup
tarsnap: stat(/home/td/new-machine.keys-bad): No such file or directory
tarsnap: Cannot read key file: /home/td/new-machine.keys-bad
         but continuing anyway with dry-run.
tarsnap: Ignoring cachedir due to missing or invalid keyfile in config file.
tarsnap: Performing dry-run archival without keys
         (sizes may be slightly inaccurate)
tarsnap: Removing leading '/' from member names
td@gin: ~/src/tarsnap/build (dryrun-keyless-ignore-config)
$ ./tarsnap -c -f a0 ~/backup
tarsnap: stat(/home/td/new-machine.keys-bad): No such file or directory
tarsnap: Cannot read key file: /home/td/new-machine.keys-bad
td@gin: ~/src/tarsnap/build (dryrun-keyless-ignore-config)

[gperciva]

Updated to continue a dryrun despite any keyfile-reading error.

td@gin: ~/src/tarsnap/build (dryrun-keyless-ignore-config)
$ ./tarsnap -c -f a1 ~/backup
tarsnap: fopen(/home/td/keys-no-filesystem-read.txt): Permission denied
tarsnap: Cannot read key file: /home/td/keys-no-filesystem-read.txt
td@gin: ~/src/tarsnap/build (dryrun-keyless-ignore-config)
$ ./tarsnap -c -f a1 --dry-run ~/backup
tarsnap: fopen(/home/td/keys-no-filesystem-read.txt): Permission denied
tarsnap: Ignoring cachedir due to missing or invalid keyfile in config file.
tarsnap: Performing dry-run archival without keys
         (sizes may be slightly inaccurate)
tarsnap: Removing leading '/' from member names
td@gin: ~/src/tarsnap/build (dryrun-keyless-ignore-config)
$ ./tarsnap -c -f a1 --dry-run --keyfile ~/keys-no-filesystem-read.txt ~/backup
tarsnap: fopen(/home/td/keys-no-filesystem-read.txt): Permission denied
tarsnap: Cannot read key file: /home/td/keys-no-filesystem-read.txt
td@gin: ~/src/tarsnap/build (dryrun-keyless-ignore-config)