[Devel] new textops functions

[Daniel-Constantin Mierla]

Hello,

there are a set of new functions added to textops module:
- insert_hf(txt) - inserts the 'txt' as header before the first header 
of the SIP message
- insert_hf(txt, hdr) - inserts the 'txt' as header before the first 
'hdr' header of the SIP message
- append_hf(txt, hdr) - adds the 'txt' as header after the first 'hdr' 
header of the SIP message

The 'txt' parameter can contain pseudo-variables which will be replaced 
at run time.

For the existing function 'append_hf(txt)' was added support for 
pseudo-variables in 'txt' parameter as well. This functions adds the 
'txt' as header after the last header of the SIP message.

Note that if some other functions inserts headers in different positions 
(e.g., record_route()), so the result will be according to the order of 
using such functions.

To give an example, if you want to add a header composed from src-ip and 
src port just after From header:

append_hf("From-addr: $si:$sp\r\n", "From");

Updated documentation for textops module:
 http://openser.org/docs/modules/1.1.x/textops.html

The list with pseudo-variables:
http://openser.org/docs/pseudo-variables-1.1.x.html

Cheers,
Daniel