Copy complete utsname if possible

Let's store a copy of the complete utsname in kdump_ctx, if available
for the given file format.

Signed-off-by: Petr Tesarik <ptesarik@suse.cz>
Add accessors to utsname members

Make it possible to read utsname members from kdump_ctx.

Signed-off-by: Petr Tesarik <ptesarik@suse.cz>
Let kdump_fdopen return the allocated context

When kdump_fdopen succeeds, it does not return the fully initialised
context variable. This bug prevents any use of the library. :-(

Signed-off-by: Petr Tesarik <ptesarik@suse.cz>