cmd/camget: Support restoring symbolic links when writing to filesystem.

This patch adds support for restoring symbolic links when camget is
called with the -o option.

This fixes an issue which caused `camget -o' to write an incomplete
snapshot to the target directory when it encountered a symlink.

Note that no attempt is made to restore the symlink metadata since: a)
A symlink's permissions do not matter and are fixed to 1777 on some
systems b) Go's os.Chtimes() always acts on the symlink target c)
os.Chown() is not currently called for any file in the current code.

Tested on Linux.

Change-Id: I7099592dcd5cc513c2685671be6f6b165f8f8053
Merge "cmd/camget: Support restoring symbolic links when writing to filesystem."
Inital support for camliContentImage.

Change-Id: Ie89d296eb1ce5b8ae4ff4d4cb8c1f97f09a4c98c
Merge "Inital support for camliContentImage."
fs: avoid possible nil pointer crash

http://camlistore.org/issue/401

Change-Id: I7ae091798e2f250898f14ec325a89cbfad4ee6aa