c-ares version 1.21.0

This is a bugfix and cleanup release with some significant internal changes.

Changes:
 o Provide better man page cross-links. [1]
 o Introduce ares_status_t as an enum rather than using #define list and
   integer data type for internal functions. [2]
 o Introduce ares_bool_t datatype rather than using an integer with 0/1 so
   it is clear based on the function prototype what it returns. [5]
 o Increase compiler warning levels by default. [6]
 o Use size_t and other more proper datatypes internally (rather than int). [7]
 o Many developers have used different code styles over the years, standardize
   on one and use clang-format to enforce the style. [8]
 o CMake can now control symbol visibility [9]
 o Replace multiple DNS hand-made parsers with new memory-safe DNS message
   parser. [10]

Bug Fixes:
 o Tools: STAYOPEN flag could make tools not terminate. [3]
 o Socket callbacks were passed SOCK_STREAM instead of SOCK_DGRAM on udp. [4]

Thanks go to these friendly people for their efforts and contributions:
  Brad House (@bradh352)
  Daniel Stenberg (@bagder)
  Gregor Jasny (@gjasny)
(3 contributors)

References to bug reports and discussions on issues:
 [1] = https://github.com/c-ares/c-ares/pull/565
 [2] = https://github.com/c-ares/c-ares/pull/567
 [3] = https://github.com/c-ares/c-ares/pull/569
 [4] = https://github.com/c-ares/c-ares/commit/a070d78
 [5] = https://github.com/c-ares/c-ares/pull/570
 [6] = https://github.com/c-ares/c-ares/pull/568
 [7] = https://github.com/c-ares/c-ares/pull/573
 [8] = https://github.com/c-ares/c-ares/pull/579
 [9] = https://github.com/c-ares/c-ares/pull/574
 [10] = https://github.com/c-ares/c-ares/pull/581