Avoid unconditional definition of _LARGEFILE_SOURCE

On some build systems (i.e. Buildroot), _LARGEFILE_SOURCE is defined
at "higher level" as default CFLAG definition.
This fix avoids the following compilation warning:

mongoose-5.4/mongoose.c:52:0: warning: "_LARGEFILE_SOURCE" redefined [enabled by default]
 #define _LARGEFILE_SOURCE       // Enable fseeko() and ftello() functions
 ^
<command-line>:0:0: note: this is the location of the previous definition

Populate HTTP warning codes

Includes changing 201 "Server Error" to "Internal Server Error"
List mostly borrowed from here http://www.iana.org/assignments/http-status-codes/http-status-codes.xhtml
418 and 451 also added
Local testing showed a problem with 202 reported as "Server Error" instead of "Accepted"

stat - function result cache

Set the correct json content-type

Chance text/json to application/json to comply with the RFC.

See http://www.ietf.org/rfc/rfc4627.txt for more information.