text-html-procs.tcl

- fixing a probably old bug: changed the markers in ad_text_to_html since the old markers with the funy camel case led to problems (at least with current regexp implementations)
in the text below, only 5 of the 8 urls were correctly transformed into anchor markup. The new markers uses uniqe single char starting and ending marker, marking also regexp much simpler.
- the function is further generalized (unifying http/https/ftp urls with mailto urls) and probably faster

A welcome contribution would be e.g. an extension of the api-browser for callbacks (listing contracts/implementations, etc.).

[1] http://openacs.org/api-doc/proc-view?proc=callback&source_p=1
[2] https://github.com/openacs/openacs-core/blob/oacs-5-9/packages/search/tcl/search-procs.tcl#L309
[3] https://github.com/openacs/forums/blob/master/tcl/forums-callback-procs.tcl#L104
[4] https://github.com/openacs/openacs-core/blob/oacs-5-9/packages/acs-tcl/tcl/apm-procs.tcl
[5] http://openacs.org/api-doc/proc-view?source_p=1&proc=apm_arg_names_for_callback_type
[6] https://github.com/openacs/xowiki/blob/oacs-5-9/tcl/xowiki-callback-procs.tcl
[7] https://github.com/openacs/openacs-core/blob/oacs-5-9/packages/acs-subsite/tcl/subsite-callback-procs.tcl#L69
[8] https://github.com/openacs/xotcl-core/blob/oacs-5-9/tcl/06-param-procs.tcl#L386