setup_once.h: refactor inclusion of <unistd.h> and <sys/socket.h>

Inclusion of top two most included header files now done in setup_once.h

curl tool: make curl.h first header included in tool_setup.h

curl tool: use configuration files from lib directory

Configuration files such as curl_config.h and all config-*.h no longer exist
nor are generated/copied into 'src' directory, now thes

curl tool: use configuration files from lib directory

Configuration files such as curl_config.h and all config-*.h no longer exist
nor are generated/copied into 'src' directory, now these only exist in 'lib'
directory from where curl tool sources uses them.

Additionally old src/setup.h has been refactored into src/tool_setup.h which
now pulls lib/setup.h

The possibility of a makefile needing an include path adjustment exists.

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curl tool: reviewed code moved to tool_*.[ch] files