Resolve symbolic links with Boost.Filesystem

[Prcuvu]

This can be done by is_symlink() and read_symlink(). The only problem is whether it is necessary.

[Prcuvu]

As reported in rime/weasel#216, user directory may be placed under a symbolic link to home directory. It may cause deployment failure if the parent symbolic link is not resolved.

[lotem]

It's complicated. I cannot create an instant fix.
Changes shall be made carefully.
While trying to find a resource by file name, path is constructed by concatenating each prefix and the relative path. In some places such path strings are compared, this can be error prone (no sound proof; while I wrote the code I thought it could be dangerous): if one is resolved by following symlink, the prefix will no longer match.

[eagleoflqj]

No boost fs any more.