REORG: include: split global.h into haproxy/global{,-t}.h

REORG: include: split global.h into haproxy/global{,-t}.h

global.h was one of the messiest files, it has accumulated tons of
implicit dependencies and declares many globals that make almost all
other file include it. It managed to silence a dependency loop between
server.h and proxy.h by being well placed to pre-define the required
structs, forcing struct proxy and struct server to be forward-declared
in a significant number of files.

It was split in to, one which is the global struct definition and the
few macros and flags, and the rest containing the functions prototypes.

The UNIX_MAX_PATH definition was moved to compat.h.