Rework the auto-detection code.

Manually and automatically detected provers are no longer separate code paths. There is now a single [partial_prover] entry family in why3.conf, with an optional boolean field manual. Field exec_name has been renamed path, and same_as has been renamed name. The latter is now always mandatory, even for automatically detected provers.

The directories from the PATH environment variable are now scanned to find files that have either the same name as an exec field (modulo .exe and .bat suffixes) or have it as a prefix followed by a dash and a digit. Also, the fully qualified name of the prover is now stored into why3.conf.