Check the -o path before looking for the mnemonic (closes #87)

age encrypt and age decrypt looked at the -o path only after the
mnemonic had been obtained, so a path that could never be written
still prompted for the mnemonic or ran the mnemonic command. Both now
refuse, before the key is derived, an -o path that is a directory or a
symlink to one, whose directory is missing or is not a directory, or
that is a symlink pointing at nothing. The check creates nothing;
output still looks at the path when it writes, so the file beside the
target is made when it was before. The README names the -o path among
what is checked first.

Model: opus-5-5

README.md

@@ -126,10 +126,12 @@ the list is named by its position, never printed. Keys are derived from the
 mnemonic's words joined by single spaces, whatever whitespace is around or
 between them, so one word per line, tabs or extra spaces give the same keys.
 
-The key index, `--words`, the `--to` recipients and the file `age encrypt` or
-`age decrypt` is given to read are checked before the mnemonic is looked for, so
-a run refused for one of them never prompts for the mnemonic or runs the
-mnemonic command.
+The key index, `--words`, the `--to` recipients, the file `age encrypt` or
+`age decrypt` is given to read and its `-o` path are checked before the mnemonic
+is looked for, so a run refused for one of them never prompts for the mnemonic
+or runs the mnemonic command. An `-o` path is refused at that point if it is a
+directory or a symlink to one, if its directory is missing or is not a
+directory, or if it is a symlink that points at nothing.
 
 `KEYFUNC_MNEMONIC` and `KEYFUNC_MNEMONIC_COMMAND` are removed from the
 environment before the mnemonic command, the system `ssh` (`keyfunc ssh to`) and