Refs #21231 -- Minor improvements and refactorings

The MAX_URLENCODED_POST_VALUES and MAX_MULTIPART_POST_PARTS settings
are renamed to URLENCODED_POST_MAX_FIELDS and MULTIPART_POST_MAX_PARTS
respectively -- the new names are more readable.

Changed the way part counting is done. The new approach has the
advantage that it will raise "SuspiciousOperation" even in case of
malicious request that contains lots of "RAW" (non FIELD/FILE) parts,
preventing excessive CPU consumption for handling malformed data.