[multiple] employ ck_calloc, ck_malloc shared code

employ ck_calloc(), ck_malloc() shared code to slightly reduce code size
(centralize the ck_assert() to check that memory allocation succeeded)

[mod_auth] save letter-case diff in require config

(thx dirk4000)

Storing the config list into a data structure with case-insensitive keys
meant that if the config list contained multiple entries w

[mod_auth] save letter-case diff in require config

(thx dirk4000)

Storing the config list into a data structure with case-insensitive keys
meant that if the config list contained multiple entries which differed
in case-only, then only one entry would survive. Case-sensitivity of
username matters for HTTP Digest auth. Store config list in value list.

x-ref:
"mod_auth (configuration): Change of behavior in user name handling"
https://redmine.lighttpd.net/boards/2/topics/10275

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[mod_auth*] rename http_auth.* -> mod_auth_api.*

rename http_auth.[ch] -> mod_auth_api.[ch]