portable: rework SHA detection Simply the SHA detection by not predicating on libcrypto, but instead checking individual header files.

portable: remove sha1.h; found portably Remove sha1.h as this is found portably across systems.

include sha2.h too where sha1.h is included In preparation for wide sha256 support; stsp@ agrees. Change done mechanically with find . -iname \*.[cy] -exec sam {} + X ,x/<sha1\.h>/i/\n#include <sha2.h>

avoid copying reused deltas to delta cache file; copy from pack file instead ok op@

let callers of got_pack_create() configure rate-limiting of progress reporting Needed by future gotd(8), where progress reports will be sent to a network socket, rather than a local terminal.

portable: fixup portably-included headers Remove certain headers which are included portably.

move privsep code from pack_create.c into new file pack_create_privsep.c Needed by future gotd(8). ok op@