httpd: support for http POST requests (API changes - step 2)

git-svn-id: svn+ssh://svn.code.sf.net/p/opensips/svn/trunk@9825 689a6050-402a-0410-94f2-e92a70836424
httpd: new parameter post_buf_size
 - this parameter will set the size of the buffer to be used
   for handling POST HTTP requests

git-svn-id: svn+ssh://svn.code.sf.net/p/opensips/svn/trunk@9828 689a6050-402a-0410-94f2-e92a70836424
httpd: basic infrastructure for POST HTTP requests

git-svn-id: svn+ssh://svn.code.sf.net/p/opensips/svn/trunk@9830 689a6050-402a-0410-94f2-e92a70836424
httpd: support for simple POST HTTP requests completed

git-svn-id: svn+ssh://svn.code.sf.net/p/opensips/svn/trunk@9835 689a6050-402a-0410-94f2-e92a70836424
httpd: documentation updates

git-svn-id: svn+ssh://svn.code.sf.net/p/opensips/svn/trunk@9848 689a6050-402a-0410-94f2-e92a70836424
PT-501: merging httpd changes in to support mi_xmlrpc_ng