package: hy_package_get_hdr_end().

To download headers only we need to know how long they are. The start is not
actually at zero, it is preceded with sig-header, but it's small (up to 2kb) and
can be used to verify the header itself.

Also, current libsolv does not expose SOLVABLE_HEADERSTART.
subject: work with full reldeps (containing the CMP flags).
selectors: allow selecting provides with full Reldep string.
New version: 0.4.9
build: 0.4.9-1
subject: work with full reldeps
moved subject and nevra from python to C