Allow absolute paths to prover plugins

[atomb]

Previously, the /proverDll option allowed only relative paths, since it looked for / or \ within a name, but not at the beginning, to determine whether the given name was a filename or a portion of an internal DLL name. This changes it to also interpret a name as a filename when either of those characters is the first character.

[shazqadeer]

I understand that on a Linux/MacOS machine, a full path starts with /. What is the appropriate way to identify a full path on Windows?

[atomb]

Most full paths on Windows will start with drive names (e.g., c:\dir1\dir2\plugin.dll), and those would already be allowed before this change. However, I believe that Windows paths starting with a backslash are valid, and that network paths are usually formatted that way, so allowing an initial backslash seems worthwhile.