Reformat FEN construction

Simplify and shrink code.

No functional change.
Teach file_to_char() about upper/lower case

This allows to further simplify Position::fen()

No functional change.
Don't exit if unable to find bench file

Now that we can call 'bench' command also from
interactive terminal it makes no more sense to
exit the application if the user types a wrong
file name.

No functional change.
Retire 'mate in x' hack

Sometimes is faster, but not always and on very long mates
produces strange scores probably due to truncation of PV
artifacts.

So simply perform normal search also in case of UCI 'mate x'
command, with the only difference that when a mate in x is
found search returns immediately.

No functional change.
Retire 'Cowardice' and 'Aggressiveness' UCI options

They are not self-describing and create a lot of user
requests about them.

Given that the values are already well tuned there
is no need to expose them as UCI options.

No functional change.