shp2pgsql usage touchups

[mwtoews]

Fixes to the usage output from shp2pgsql.

1. the "-w" flag is not documented here, but is documented in using_postgis_dataman.xml (the flag is still accepted by the program, although see #603). I've reworded this, so please read-over my change.
2. A few missing "."'s at the end (for consistancy)
3. An unmatched ")"
4. Align a few segments of text.

I have a patch for trunk, and also for 1.5.3 (if it isn't too late). However you might want to consider not adding the "-w" flag for the 1.5 branch due to #603 (unless that is an easy fix).

[mwtoews]

Just updated the patches, as they had "dirfts" .. not "drifts"

[pramsey]

Committed to 1.5 at r6373. Committed to trunk at r6374.

I didn't commit the trunk version in its entirety as I didn't agree with all the punctuation. And I think I would rather remove the -w option than document it.