Fix Proxy-Protocol log field symbols

[masaori335]

What

Log::init_fields() registered the Proxy Protocol source/destination IP log
fields into field_symbol_hash under ppsip/ppdip, but the LogFields
(and the documentation) use the symbols pps/ppd. This aligns the hash keys
with the actual symbols.

Why this is not a critical bug

Obviously this is a bug, but logging output was not broken. Because
LogFieldList::find_by_symbol() falls back to a linear scan by symbol on a
hash miss, so %<pps>/%<ppd> still resolved correctly.

The mismatch only meant:

• the O(1) hash fast-path was skipped for these two fields (resolved via the
  slower fallback instead),
• the undocumented ppsip/ppdip aliases were silently accepted, and
• the hash-only conflict/replace check in TSLogFieldRegister() could
  mis-handle the pps/ppd symbols.

Note: formats that relied on the bogus ppsip/ppdip aliases will now be
rejected as unknown symbols; use the documented pps/ppd.

[bryancall]

Looks good