support Linux and macOS curl binaries

Add the ability to create Linux and macOS binaries. We are keeping the name curl-for-win though. Non-Windows binaries are experimental at this point and they are not official curl binaries for these platforms.

Highlights:

add support for Linux builds (both clang and gcc).
add support for Linux cross-CPU (arm64-x86_64) builds with glibc.
add support for Linux MUSL builds, both clang and gcc, hosted on either Alpine, Debian bookworm / bullseye / trixie.
use full relro, _FORTIFY_SOURCE=2 and -fstack-protector-all for Linux.
add support for Linux MUSL cross-built from macOS. (experimental)
add support for macOS builds (hosted on macOS), both llvm-clang and Apple clang. These also feature LDAP/LDAPS and SecureTransport support.
add support for Universal macOS builds.
dump info about the built Linux and macOS binaries.
add new GitHub Actions to builds a set of selected combinations, both daily and per-commit. This is subject to change.
enable ASM support for ARM64 with BoringSSL.
add hack to build x64 Windows again with BoringSSL.
add support for AWS-LC (BoringSSL-like) TLS backend. (it requires MSVC, so Windows not supported for this backend.)
add support for bldtst builds that use no 3rd-party dependencies except zlib. To make build tests/porting easier.
add spuport for nozlib option to fully opt-out from 3rd-party dependencies when used together with bldtst.
fix curl autotools builds without any TLS backend.
fix compatibility with bash v3.x.
fix scripts for busybox.
remove dependence on dos2unix and unix2dos tools.
remove dependence on realpath command.
fixup deleting our local gpgdir on abort.

Caveats:

This is an ongoing effort. There is a lot to explore and complete, some of these listed in the TODO section of _build.sh.
Linux MUSL cannot be cross-built yet for other CPUs.
libcurl static and shared libs build without errors, but they were not functionally tested yet.
Due to weirdness and possibly bugs in the Xcode toolchain, reproducible builds are not yet possible for macOS. (unless using llvm/clang.)

Linux:

Protocols: dict file ftp ftps gopher gophers http https imap imaps            mqtt pop3 pop3s rtsp scp sftp smb smbs smtp smtps telnet tftp ws wss
Features: alt-svc AsynchDNS brotli       HSTS HTTP2 HTTP3 HTTPS-proxy     IPv6          Largefile libz          NTLM            SSL      threadsafe UnixSockets zstd

macOS (quictls + SecureTransport):

Protocols: dict file ftp ftps gopher gophers http https imap imaps ldap ldaps mqtt pop3 pop3s rtsp scp sftp smb smbs smtp smtps telnet tftp ws wss
Features: alt-svc AsynchDNS brotli       HSTS HTTP2 HTTP3 HTTPS-proxy     IPv6          Largefile libz MultiSSL NTLM            SSL      threadsafe UnixSockets zstd

Closes #53

Merge without squashing!

curl / curl-for-win

support Linux and macOS curl binaries #53