Add ap_varbuf API for resizable buffers.

Increase length limit of lines in the configuration file to 16MB.
Increase length limit of lines in the group file to 16MB.

PR: 45888, 50824, 43084

Windows and Netware build changes are untested.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1157354 13f79535-47bb-0310-9956-ffa450edef68