Prepares for sharing code between got-index-pack and future gotd(8).

ok millert@

Needed by future gotd(8), where progress reports will be sent to a network socket, rather than a local terminal.

For all key Makefile.am files in use, sort the files so that they're more easily identifiable -- especially when adding/removing source files.

Remove certain headers which are included portably.

Needed by future gotd(8). ok op@

ok op@

ok op@

ok op@

ok op@

ok op@

Future gotd(8) needs to run got_pack_create() in a chroot environment, so we can no longer open new temporary files inside got_pack_create(). ok op@

The old code required a seekable output file. This conflicts with requirements of future gotd(8), which will write pack file data to network sockets. ok op@

Also add basic test coverage for reading patches from stdin, while here. ok op@

Unfortunately, MacOS doesn't provide sys/tree.h -- so the compat check for this must happen portably.

For those tests which rely on compilation files, order them alphabetically so it's easier when having to change the makefile stanzas.

When checking for sys/tree.h on non-BSD systems, either it will be in the standard include path (such as on MacOS), or it won't be found, until libbsd is overlayed. In cases where we want to find sys/tree.h via libbsd, perform a AC_CHECK_DECL for a known #define in sys/tree.h -- which will still find sys/tree.h in either location, or uses compat/tree.h in all other cases.

This is searched for portably.