sockopts: support many more socket options, including strings

[robbat2]

Add support for many more socket options, including those that take string inputs, such as TCP_CONGESTION=bbr.

Options known to the code, but not present at build time will now emit an error that is distict from options unknown to the code (unknown vs not-available).

This patch greatly eases running specific rsync configurations, without relying on LD_PRELOAD to modify socket behaviors.

[robbat2]

maintainer:
It's not clear to me why the FreeBSD build cannot find the new symbol. The Linux build finds it correctly, even with as-needed.

./configure -C --enable-maintainer-mode CFLAGS="$CFLAGS -Wall -W -Wa,--noexecstack -Wl,--as-needed -O3 -ggdb3" && make clean && gmake -j && strace -e trace=setsockopt ./rsync --sockopts='SO_INCOMING_CPU=1,IP_FREEBIND,IPTOS_RELIABILITY,TCP_FASTOPEN,TCP_FASTOPEN_CONNECT'  localhost::

...
gcc -I./zlib  -Wall -W -Wa,--noexecstack -Wl,--as-needed -O3 -ggdb3 -DMAINTAINER_MODE -DHAVE_CONFIG_H -Wall -W  -o rsync flist.o rsync.o generator.o receiver.o cleanup.o sender.o exclude.o util1.o util2.o main.o checksum.o match.o syscall.o log.o backup.o delete.o options.o io.o compat.o hlink.o token.o uidlist.o socket.o hashtable.o usage.o fileio.o batch.o clientname.o chmod.o acls.o xattrs.o progress.o pipe.o sockopts.o  simd-checksum-x86_64.o  params.o loadparm.o clientserver.o access.o connection.o authenticate.o lib/wildmatch.o lib/compat.o lib/snprintf.o lib/mdfour.o lib/md5.o lib/permstring.o lib/pool_alloc.o lib/sysacls.o lib/sysxattrs.o  zlib/deflate.o zlib/inffast.o zlib/inflate.o zlib/inftrees.o zlib/trees.o zlib/zutil.o zlib/adler32.o zlib/compress.o zlib/crc32.o  -lattr -lacl -lpopt -llz4 -lzstd -lxxhash -lcrypto 
setsockopt(3, SOL_SOCKET, SO_INCOMING_CPU, [1], 4) = 0
setsockopt(3, SOL_IP, IP_FREEBIND, [1], 4) = 0
setsockopt(3, SOL_IP, IP_TOS, [4], 4)   = 0
setsockopt(3, SOL_TCP, TCP_FASTOPEN, [1], 4) = 0
setsockopt(3, SOL_TCP, TCP_FASTOPEN_CONNECT, [1], 4) = 0
rsync-test-module	
+++ exited with 0 +++

[robbat2]

@WayneD this is ready for your review

[tridge]

@robbat2 needs a rebase

[robbat2]

@tridge rebased now; only conflict was in NEWS.md ; verified that the tests also still pass

[robbat2]

@tridge rebased again if you could merge please?

[robbat2]

@WayneD @tridge merge please? rebased again for NEWS.md changes