Patch by Josiah Frentsos, thanks!

This adds a set of functions to abstract over SHA1Init, SHA1Update, SHA1Final, their respective SHA256 variants and how to compare digests. Replace all the SHA1*() usage with the new APIs. It's a preparatory step for sha256 handling. ok stsp@

It will soon grow functions to deal with sha256 too. stsp@ agrees.

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>

The parent no longer sends this message. Perform related cleanup in the shutdown path instead. ok op@

ok stsp@

Allow clients to run "got send -d" against gotd. Clients will send a zero-id as new id for a reference and, in the special but more common case of just deleting and not updating, no pack file will be sent. ok and tweaks by stsp@

Clients will send an empty pack file if they are only creating new references and have no objects to upload. Make gotd handle this and add a regression test which triggers the bug. Problem found by op@. The new regression test caught an unrelated issue where the client connection was left lingering after references had been updated, which made 'got send' followed by 'got clone -l' fail with the connection limit configured for the test suite (just one connection is allowed at a time). Fix this as well. ok op@

ok stsp@

The new session process is able to manipulate files in the repository and keeps track of the read/write client session state. The parent process now restricts its view of the filesystem to the absolute path stored in argv[0], and combines this with unveil "x" on this path. As a result the parent process can only re-exec itself. small tweaks + ok op@

Because these processes are now started on demand per client connection there is no need to keep track of multiple clients anymore. Also, these processes can now exit when a disconnect event is received. ok op, jamsek

ok op, jamsek