MINOR: 51d: unable to start haproxy without "51degrees-data-file"

MINOR: 51d: unable to start haproxy without "51degrees-data-file"

This patch adds a few checks on "global._51degrees.data_file_path" and allows
haproxy to start even when the pattern or trie data file is not specified.

If the "51d" converter is used, a new function "_51d_conv_check" will check
"global._51degrees.data_file_path" and displays a warning if necessary.

In src/haproxy.c, the global 51Degrees "cache_size" has moved outside of the
FIFTYONEDEGREES_H_PATTERN_INCLUDED ifdef block.