Reduce header dependencies.

Seeking on NFS or SMB is cheap.  Actually, only HTTP streams are
expensive to seek.  This enables a few features on NFS/SMB files, for
example Ogg tags.

Has been moved to class InputStream long ago.

Since we already checked InputStream::KnownSize(), we can assume that
GetSize() returns a valid value, and this check is obsolete.

Replace the bogus GetSize() check and call GetSize() only when
necessary.

Variable-length arrays are not allowed in C++.

If the offset is out of range, the seek will simply fail.  Not a
problem.

The plugin assumes that the size is known, but does not verify it at
runtime.

Generate more detailed error messages.

Eliminate duplicate definition (in input plugin and decoder plugin).

Again, POSIX says strcasecmp is not in string.h.

According to POSIX and both OSX and Linux manpages,
strcasecmp comes from strings.h, not string.h.

Most OSes also have them available in string.h,
but we just fixed the headers on Haiku and it now
only provides them in strings.h.

We might want to fall back to string.h for other
OSes though...

cf.
http://pubs.opengroup.org/onlinepubs/009695399/functions/strcasecmp.html
http://linux.die.net/man/3/strcasecmp
https://developer.apple.com/library/mac/documentation/Darwin/Reference/ManPages/man3/strcasecmp.3.html