Adjust chat log opening functions to use LLLogChat functions to determine log file

This eliminates the possibility of the history button not opening the log file, which has been reported in the past.
This also corrects cases where log files named with date would not be opened.
User-configurable log filename date formatting (When using "Append date to log filename")

The old defaults remain the same, so no worries for old users of this feature
Modify LogFileLocalChatDateFormat and LogFileIMsDateFormat in debug settings, use strftime's format specifiers.
The above mentioned settings are per account.

Inspired by irssi
Fix Issue 1012: Private voice call

The original purpose of the patch removed here was to fix this very bug, though I could not reproduce it at that time...
There must still be an edge case where this bug will reproduce but I tweaked the involved settings around quite a bit and could not reproduce it