Check whether the logfile is a regular file. Fixed incomplete privilege drop.

[m0n73]

If the requested log file (-l option) isn't a regular file, fopen will fail to open it, subsequently passing a NULL pointer into fclose which causes a segfault. This patch adds an appropriate file type check and shifts the logic around, so that we only create a new log file if it doesn't already exist.

[m0n73]

Adding another commit to this pull request, which fixes incomplete privilege drop in perform_user_change().