add --tags to 'git describe'

'git describe' calculates the version since the last **annotated** diff. Adding --tags shows the most recent tags (v0.94, for example) regardless if they're annotated or not.
put git version detection code into a sub
Merge branch 'version' into qpsmtpd.t
simplify config->from_file by not requiring full

file path. Instead, figure it out the same way as the 4 of the 5 caller do (by calling config_dir). Allow the 2nd arg to be the file name.