allows pv_format specifier in prefix param

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#### Description <!-- Describe your changes in detail --> add `prefix_mode` param to xlog to optionally interpret `prefix` as a `pv_format` specifier.

`config`: ``` mod_param("xlog", "prefix", "$_s([$(cfg(file):$cfg(line)] $ci $(cfg(route){s.tolower}) )") mod_param("xlog", "prefix_mode", 1) ... xlog("L_INFO", "hello") ... ``` `output:` <pre><code> 66(307) INFO: <b>[my-file.cfg:5]</b> 62795aa2-9696c27c <b>my_route</b> hello </code> </pre>

You can view, comment on, or merge this pull request online at:

https://github.com/kamailio/kamailio/pull/2845

-- Commit Summary --

* xlog: add prefix_mode

-- File Changes --

M src/modules/xlog/doc/xlog_admin.xml (22) M src/modules/xlog/xlog.c (46)

-- Patch Links --

https://github.com/kamailio/kamailio/pull/2845.patch https://github.com/kamailio/kamailio/pull/2845.diff