autotools: cleanup usage of MSG_NOSIGNAL

This is Part of an autotools buildsystem port.

Previously MSG_NOSIGNAL was defined in a few places, and -DMSG_NOSIGNAL=0 was
passed by the osx build to signify that it was unavailable there. For Win32, it
was assumed to be unavailable.

This commit flips the logic so that the unix build declares the feature. The
HAVE_MSG_NOSIGNAL define will be (automatically detected and) set by autotools
as well, so that both build methods will give the same results.

There should be no observable changes in functionality here.
autotools: include bitcoin-config.h for autotools builds
autotools: add gitignore
autotools: add autotools files
gitian: path fix
Merge remote-tracking branch 'theuni/autotools-rfc-1' into gitian-cleanup