portable: OpenBSD compilation fixes

portable: fixes for NetBSD * Move the main incldue/got_compat.h to the top of the files they're included in. * Define SHA256Init and friends to the system equivalents.

portable: cleanup headers Remove useless headers which don't need to be included any longer.

portable: improve b64_ntop detection When looking for base64 functions, check -lresolv and/or -lnetwork, and only build those in compat if required. Some of the detection code was modified from tmux's configure.ac script.

portable: add missing header checks Some code in compat/ has #defines which were not being checked for in configure.ac, which could have lead to compilation failures were those files to be included on systems which needed them. In doing so, we can now remove libmd and libcrypto as these are no longer needed.

portable: configure: use SHA_2{,56}_H Don't use a custom variable when the header checks for these are already available. From Christian "naddy" Weisgerber.

portable: alpine: include stdlib.h Because musl is less forgiving about header-ordering, explicitly include this before stdio.h -- this won't affect existing files as the include guards elsewhere prevent this.

portable: extend got_compat2 include guard Extend the include guard to the whole file.

portable: configure: split out dependencies Rather than assume all dependencies are required for all programs, split them out. This will make packaging easier, as well as splitting the code to use subprojects. Note that due to the use of config.h semantics, in most cases the got_compat.h header file is now at the top of the .c file it is included in, so that it can handle the system header inclusion properly.

portable: configure: use config.h So that the command-line flags to CC don't get drowned out with copious -D flags, move this detection to config.h instead. Note that to reduce the code churn, the existing include/got_compat.h header file has been renamed, and the generated file replaces it, but with the contents of the original header file included. Eventually, we might want to move some od the logic in include/got_compat2.h into AH_VERBATIM() blocks, but for now, this is a good compromise.